/*++
Copyright (c) 2017 Arie Gurfinkel

Module Name:

    spacer_proof_utils.cpp

Abstract:
    Utilities to traverse and manipulate proofs

Author:
    Bernhard Gleiss
    Arie Gurfinkel

Revision History:

--*/

#include "util/params.h"
#include "ast/ast_pp.h"
#include "ast/ast_util.h"
#include "ast/proofs/proof_checker.h"
#include "muz/base/dl_util.h"
#include "muz/spacer/spacer_iuc_proof.h"

#include "ast/proofs/proof_utils.h"
#include "muz/spacer/spacer_proof_utils.h"
#include "muz/spacer/spacer_util.h"

namespace spacer {

    // arithmetic lemma recognizer
    bool is_arith_lemma(ast_manager& m, proof* pr)
    {
        // arith lemmas: second parameter specifies exact type of lemma,
        // could be "farkas", "triangle-eq", "eq-propagate",
        // "assign-bounds", maybe also something else
        if (pr->get_decl_kind() == PR_TH_LEMMA) {
            func_decl* d = pr->get_decl();
            symbol sym;
            return d->get_num_parameters() >= 1 &&
                d->get_parameter(0).is_symbol(sym) &&
                sym == "arith";
        }
        return false;
    }

    // farkas lemma recognizer
    bool is_farkas_lemma(ast_manager& m, proof* pr)
    {
        if (pr->get_decl_kind() == PR_TH_LEMMA)
        {
            func_decl* d = pr->get_decl();
            symbol sym;
            return d->get_num_parameters() >= 2 &&
                d->get_parameter(0).is_symbol(sym) && sym == "arith" &&
                d->get_parameter(1).is_symbol(sym) && sym == "farkas";
        }
        return false;
    }

    static bool is_assign_bounds_lemma(ast_manager &m, proof *pr) {
        if (pr->get_decl_kind() == PR_TH_LEMMA)
        {
            func_decl* d = pr->get_decl();
            symbol sym;
            return d->get_num_parameters() >= 2 &&
                d->get_parameter(0).is_symbol(sym) && sym == "arith" &&
                d->get_parameter(1).is_symbol(sym) && sym == "assign-bounds";
        }
        return false;
    }



    class linear_combinator {
        struct scaled_lit {
            bool is_pos;
            app *lit;
            rational coeff;
            scaled_lit(bool is_pos, app *lit, const rational &coeff) :
                is_pos(is_pos), lit(lit), coeff(coeff) {}
        };
        ast_manager &m;
        th_rewriter m_rw;
        arith_util m_arith;
        expr_ref m_sum;
        bool m_is_strict;
        rational m_lc;
        vector<scaled_lit> m_lits;
    public:
        linear_combinator(ast_manager &m) : m(m), m_rw(m), m_arith(m),
                                            m_sum(m), m_is_strict(false),
                                            m_lc(1) {}

        void add_lit(app* lit, rational const &coeff, bool is_pos = true) {
            m_lits.push_back(scaled_lit(is_pos, lit, coeff));
        }

        void normalize_coeff() {
            for (auto &lit : m_lits)
                m_lc = lcm(m_lc, denominator(lit.coeff));
            if (!m_lc.is_one()) {
                for (auto &lit : m_lits)
                    lit.coeff *= m_lc;
            }
        }

        rational const &lc() const {return m_lc;}

        bool process_lit(scaled_lit &lit0) {
            arith_util a(m);
            app* lit = lit0.lit;
            rational &coeff = lit0.coeff;
            bool is_pos = lit0.is_pos;


            if (m.is_not(lit)) {
                lit = to_app(lit->get_arg(0));
                is_pos = !is_pos;
            }
            if (!m_arith.is_le(lit) && !m_arith.is_lt(lit) &&
                !m_arith.is_ge(lit) && !m_arith.is_gt(lit) && !m.is_eq(lit)) {
                return false;
            }
            SASSERT(lit->get_num_args() == 2);
            sort* s = m.get_sort(lit->get_arg(0));
            bool is_int = m_arith.is_int(s);
            if (!is_int && m_arith.is_int_expr(lit->get_arg(0))) {
                is_int = true;
                s = m_arith.mk_int();
            }

            if (!is_int && is_pos && (m_arith.is_gt(lit) || m_arith.is_lt(lit))) {
                m_is_strict = true;
            }
            if (!is_int && !is_pos && (m_arith.is_ge(lit) || m_arith.is_le(lit))) {
                m_is_strict = true;
            }


            SASSERT(m_arith.is_int(s) || m_arith.is_real(s));
            expr_ref sign1(m), sign2(m), term(m);
            sign1 = m_arith.mk_numeral(m.is_eq(lit)?coeff:abs(coeff), s);
            sign2 = m_arith.mk_numeral(m.is_eq(lit)?-coeff:-abs(coeff), s);
            if (!m_sum.get()) {
                m_sum = m_arith.mk_numeral(rational(0), s);
            }

            expr* a0 = lit->get_arg(0);
            expr* a1 = lit->get_arg(1);

            if (is_pos && (m_arith.is_ge(lit) || m_arith.is_gt(lit))) {
                std::swap(a0, a1);
            }
            if (!is_pos && (m_arith.is_le(lit) || m_arith.is_lt(lit))) {
                std::swap(a0, a1);
            }

            //
            // Multiplying by coefficients over strict
            // and non-strict inequalities:
            //
            // (a <= b) * 2
            // (a - b <= 0) * 2
            // (2a - 2b <= 0)

            // (a < b) * 2       <=>
            // (a +1 <= b) * 2   <=>
            // 2a + 2 <= 2b      <=>
            // 2a+2-2b <= 0

            bool strict_ineq =
                is_pos?(m_arith.is_gt(lit) || m_arith.is_lt(lit)):(m_arith.is_ge(lit) || m_arith.is_le(lit));

            if (is_int && strict_ineq) {
                m_sum = m_arith.mk_add(m_sum, sign1);
            }

            term = m_arith.mk_mul(sign1, a0);
            m_sum = m_arith.mk_add(m_sum, term);
            term = m_arith.mk_mul(sign2, a1);
            m_sum = m_arith.mk_add(m_sum, term);

            m_rw(m_sum);
            return true;
        }

        expr_ref operator()(){
            if (!m_sum) normalize_coeff();
            m_sum.reset();
            for (auto &lit : m_lits) {
                if (!process_lit(lit)) return expr_ref(m);
            }
            return m_sum;
        }
    };

/*
 * ====================================
 * methods for transforming proofs
 * ====================================
 */

    void theory_axiom_reducer::reset() {
        m_cache.reset();
        m_pinned.reset();
    }

    static proof_ref  mk_th_lemma(ast_manager &m, ptr_buffer<proof> const &parents,
                                  unsigned num_params, parameter const *params) {
        buffer<parameter> v;
        for (unsigned i = 1; i < num_params; ++i)
            v.push_back(params[i]);

        SASSERT(params[0].is_symbol());
        family_id tid = m.mk_family_id(params[0].get_symbol());
        SASSERT(tid != null_family_id);

        proof_ref pf(m);
        pf =  m.mk_th_lemma(tid, m.mk_false(),
                            parents.size(), parents.c_ptr(),
                            v.size(), v.c_ptr());
        return pf;
    }

    static bool match_mul(expr *e, expr_ref &var, expr_ref &val, arith_util &a) {
        expr *e1 = nullptr, *e2 = nullptr;
        if (!a.is_mul(e, e1, e2)) {
            if (a.is_numeral(e)) return false;
            if (!var || var == e) {
                var = e;
                val = a.mk_numeral(rational(1), get_sort(e));
                return true;
            }
            return false;
        }

        if (!a.is_numeral(e1)) std::swap(e1, e2);
        if (!a.is_numeral(e1)) return false;

        // if variable is given, match it as well
        if (!var || var == e2) {
            var = e2;
            val = e1;
            return true;
        }
        return false;
    }

    static expr_ref get_coeff(expr *lit0, expr_ref &var) {
        ast_manager &m = var.m();
        arith_util a(m);

        expr *lit = nullptr;
        if (!m.is_not(lit0, lit)) lit = lit0;

        expr *e1 = nullptr, *e2 = nullptr;
        // assume e2 is numeral and ignore it
        if ((a.is_le(lit, e1, e2) || a.is_lt(lit, e1, e2) ||
               a.is_ge(lit, e1, e2) || a.is_gt(lit, e1, e2) ||
               m.is_eq(lit, e1, e2))) {
            if (a.is_numeral(e1)) std::swap(e1, e2);
        }
        else {
            e1 = lit;
        }

        expr_ref val(m);
        if (!a.is_add(e1))  {
            if (match_mul(e1, var, val, a)) return val;
        }
        else {
            for (auto *arg : *to_app(e1)) {
                if (match_mul(arg, var, val, a)) return val;
            }
        }
        return expr_ref(m);
    }

    // convert assign-bounds lemma to a farkas lemma by adding missing coeff
    // assume that missing coeff is for premise at position 0
    static proof_ref mk_fk_from_ab(ast_manager &m,
                                   ptr_buffer<proof> const &parents,
                                   unsigned num_params,
                                   parameter const *params) {
        SASSERT(num_params == parents.size() + 1 /* one param is missing */);
        arith_util a(m);
        th_rewriter rw(m);

        // compute missing coefficient
        linear_combinator lcb(m);
        for (unsigned i = 1, sz = parents.size(); i < sz; ++i) {
            app *p = to_app(m.get_fact(parents.get(i)));
            rational const &r = params[i+1].get_rational();
            lcb.add_lit(p, r);
        }

        expr_ref lit0(m);
        lit0 = m.get_fact(parents.get(0));
        // put lit0 into canonical form
        rw(lit0);
        TRACE("spacer.fkab",
              tout << "lit0 is: " << lit0 << "\n"
              << "LCB is: " << lcb() << "\n";);

        expr_ref var(m), val1(m), val2(m);
        val1 = get_coeff(lit0, var);
        val2 = get_coeff(lcb(), var);
        TRACE("spacer.fkab",
              tout << "var: " << var
              << " val1: " << val1 << " val2: "  << val2 << "\n";);

        rational rat1, rat2, coeff0;
        CTRACE("spacer.fkab", !(val1 && val2),
               tout << "Failed to match variables\n";);
        if (val1 && val2 &&
            a.is_numeral(val1, rat1) && a.is_numeral(val2, rat2)) {
            coeff0 = abs(rat2/rat1);
            coeff0 = coeff0 / lcb.lc();
            TRACE("spacer.fkab", tout << "coeff0: " << coeff0 << "\n";);
        }
        else {
            IF_VERBOSE(1, verbose_stream()
                       << "\n\n\nFAILED TO FIND COEFFICIENT\n\n\n";);
            TRACE("spacer.fkab", tout << "FAILED TO FIND COEFFICIENT\n";);
            // failed to find a coefficient
            return proof_ref(m);
        }


        buffer<parameter> v;
        v.push_back(parameter(symbol("farkas")));
        v.push_back(parameter(coeff0));
        for (unsigned i = 2; i < num_params; ++i)
            v.push_back(params[i]);

        SASSERT(params[0].is_symbol());
        family_id tid = m.mk_family_id(params[0].get_symbol());
        SASSERT(tid != null_family_id);

        proof_ref pf(m);
        pf = m.mk_th_lemma(tid, m.mk_false(),
                           parents.size(), parents.c_ptr(),
                           v.size(), v.c_ptr());

        SASSERT(is_arith_lemma(m, pf));

        DEBUG_CODE(
            proof_checker pc(m);
            expr_ref_vector side(m);
            ENSURE(pc.check(pf, side)););
        return pf;

    }

    /// -- rewrite theory axioms into theory lemmas
    proof_ref theory_axiom_reducer::reduce(proof* pr) {
        proof_post_order pit(pr, m);
        while (pit.hasNext()) {
            proof* p = pit.next();

            if (m.get_num_parents(p) == 0 && is_arith_lemma(m, p)) {
                // we have an arith-theory-axiom and want to get rid of it
                // we need to replace the axiom with
                // (a) corresponding hypothesis,
                // (b) a theory lemma, and
                // (c) a lemma.
                // Furthermore update data-structures
                app *fact = to_app(m.get_fact(p));
                ptr_buffer<expr> cls;
                if (m.is_or(fact)) {
                    for (unsigned i = 0, sz = fact->get_num_args(); i < sz; ++i)
                        cls.push_back(fact->get_arg(i));
                }
                else
                    cls.push_back(fact);

                // (a) create hypothesis
                ptr_buffer<proof> hyps;
                for (unsigned i = 0, sz = cls.size(); i < sz; ++i) {
                    expr *c;
                    expr_ref hyp_fact(m);
                    if (m.is_not(cls[i], c))
                        hyp_fact = c;
                    else
                        hyp_fact = m.mk_not (cls[i]);

                    proof* hyp = m.mk_hypothesis(hyp_fact);
                    m_pinned.push_back(hyp);
                    hyps.push_back(hyp);
                }

                // (b) Create a theory lemma
                proof_ref th_lemma(m);
                func_decl *d = p->get_decl();
                if (is_assign_bounds_lemma(m, p)) {

                    th_lemma = mk_fk_from_ab(m, hyps,
                                             d->get_num_parameters(),
                                             d->get_parameters());
                }

                // fall back to th-lemma
                if (!th_lemma) {
                    th_lemma = mk_th_lemma(m, hyps,
                                           d->get_num_parameters(),
                                           d->get_parameters());
                }
                m_pinned.push_back(th_lemma);
                SASSERT(is_arith_lemma(m, th_lemma));

                // (c) create lemma
                proof* res = m.mk_lemma(th_lemma, fact);
                m_pinned.push_back(res);
                m_cache.insert(p, res);

                SASSERT(m.get_fact(res) == m.get_fact(p));
            }
            else {
                // proof is dirty, if a sub-proof of one of its premises
                // has been transformed
                bool dirty = false;

                ptr_buffer<expr> args;
                for (unsigned i = 0, sz = m.get_num_parents(p); i < sz; ++i) {
                    proof *pp, *tmp;
                    pp = m.get_parent(p, i);
                    VERIFY(m_cache.find(pp, tmp));
                    args.push_back(tmp);
                    dirty |= (pp != tmp);
                }
                // if not dirty just use the old step
                if (!dirty) m_cache.insert(p, p);
                // otherwise create new proof with the corresponding proofs
                // of the premises
                else {
                    if (m.has_fact(p)) args.push_back(m.get_fact(p));

                    SASSERT(p->get_decl()->get_arity() == args.size());

                    proof* res = m.mk_app(p->get_decl(),
                                          args.size(), (expr * const*)args.c_ptr());
                    m_pinned.push_back(res);
                    m_cache.insert(p, res);
                }
            }
        }

        proof* res;
        VERIFY(m_cache.find(pr,res));
        DEBUG_CODE(
            proof_checker pc(m);
            expr_ref_vector side(m);
            SASSERT(pc.check(res, side));
            );

        return proof_ref(res, m);
    }

/* ------------------------------------------------------------------------- */
/* hypothesis_reducer */
/* ------------------------------------------------------------------------- */

    proof_ref hypothesis_reducer::reduce(proof* pr) {
        compute_hypsets(pr);
        collect_units(pr);

        proof_ref res(reduce_core(pr), m);
        SASSERT(res);
        reset();

        DEBUG_CODE(proof_checker pc(m);
                   expr_ref_vector side(m);
                   SASSERT(pc.check(res, side)););
        return res;
    }

    void hypothesis_reducer::reset() {
        m_active_hyps.reset();
        m_units.reset();
        m_cache.reset();
        for (auto t : m_pinned_active_hyps) dealloc(t);
        m_pinned_active_hyps.reset();
        m_pinned.reset();
        m_hyp_mark.reset();
        m_open_mark.reset();
        m_visited.reset();
    }

    void hypothesis_reducer::compute_hypsets(proof *pr) {
        ptr_buffer<proof> todo;
        todo.push_back(pr);

        while (!todo.empty()) {
            proof* p = todo.back();

            if (m_visited.is_marked(p)) {
                todo.pop_back();
                continue;
            }

            unsigned todo_sz = todo.size();
            for (unsigned i = 0, sz = m.get_num_parents(p); i < sz; ++i) {
                SASSERT(m.is_proof(p->get_arg(i)));
                proof *parent = to_app(p->get_arg(i));

                if (!m_visited.is_marked(parent))
                    todo.push_back(parent);
            }
            if (todo.size() > todo_sz) continue;

            todo.pop_back();

            m_visited.mark(p);


            proof_ptr_vector* active_hyps = nullptr;
            // fill both sets
            if (m.is_hypothesis(p)) {
                // create active_hyps-set for step p
                proof_ptr_vector* active_hyps = alloc(proof_ptr_vector);
                m_pinned_active_hyps.insert(active_hyps);
                m_active_hyps.insert(p, active_hyps);
                active_hyps->push_back(p);
                m_open_mark.mark(p);
                m_hyp_mark.mark(m.get_fact(p));
                continue;
            }

            ast_fast_mark1 seen;

            active_hyps = alloc(proof_ptr_vector);
            for (unsigned i = 0, sz = m.get_num_parents(p); i < sz; ++i) {
                proof* parent = m.get_parent(p, i);
                // lemmas clear all hypotheses above them
                if (m.is_lemma(p)) continue;
                for (auto *x : *m_active_hyps.find(parent)) {
                    if (!seen.is_marked(x)) {
                        seen.mark(x);
                        active_hyps->push_back(x);
                        m_open_mark.mark(p);
                    }
                }
            }
            if (active_hyps->empty()) {
                dealloc(active_hyps);
                m_active_hyps.insert(p, &m_empty_vector);
            }
            else {
                m_pinned_active_hyps.push_back(active_hyps);
                m_active_hyps.insert(p, active_hyps);
            }
        }
    }

// collect all units that are hyp-free and are used as hypotheses somewhere
// requires that m_active_hyps has been computed
    void hypothesis_reducer::collect_units(proof* pr) {

        proof_post_order pit(pr, m);
        while (pit.hasNext()) {
            proof* p = pit.next();
            if (!m.is_hypothesis(p)) {
                // collect units that are hyp-free and are used as
                // hypotheses in the proof pr
                if (!m_open_mark.is_marked(p) && m.has_fact(p) &&
                    m_hyp_mark.is_marked(m.get_fact(p)))
                    m_units.insert(m.get_fact(p), p);
            }
        }
    }

/**
   \brief returns true if p is an ancestor of q
*/
    bool hypothesis_reducer::is_ancestor(proof *p, proof *q) {
        if (p == q) return true;
        ptr_vector<proof> todo;
        todo.push_back(q);

        expr_mark visited;
        while (!todo.empty()) {
            proof *cur;
            cur = todo.back();
            todo.pop_back();

            if (visited.is_marked(cur)) continue;

            if (cur == p) return true;
            visited.mark(cur);

            for (unsigned i = 0, sz = m.get_num_parents(cur); i < sz; ++i) {
                todo.push_back(m.get_parent(cur, i));
            }
        }
        return false;
    }

    proof* hypothesis_reducer::reduce_core(proof* pf) {
        SASSERT(m.is_false(m.get_fact(pf)));

        proof *res = NULL;

        ptr_vector<proof> todo;
        todo.push_back(pf);
        ptr_buffer<proof> args;
        bool dirty = false;

        while (true) {
            proof *p, *tmp, *pp;
            unsigned todo_sz;

            p = todo.back();
            if (m_cache.find(p, tmp)) {
                todo.pop_back();
                continue;
            }

            dirty = false;
            args.reset();
            todo_sz = todo.size();
            for (unsigned i = 0, sz = m.get_num_parents(p); i < sz; ++i) {
                pp = m.get_parent(p, i);
                if (m_cache.find(pp, tmp)) {
                    args.push_back(tmp);
                    dirty |= pp != tmp;
                } else {
                    todo.push_back(pp);
                }
            }

            if (todo_sz < todo.size()) continue;

            todo.pop_back();

            // transform the current proof node

            if (m.is_hypothesis(p)) {
                // if possible, replace a hypothesis by a unit derivation
                if (m_units.find(m.get_fact(p), tmp)) {
                    // use already transformed proof of the unit if it is available
                    proof* proof_of_unit;
                    if (!m_cache.find(tmp, proof_of_unit)) {
                        proof_of_unit = tmp;
                    }

                    // make sure hypsets for the unit are computed
                    // AG: is this needed?
                    compute_hypsets(proof_of_unit);

                    // if the transformation doesn't create a cycle, perform it
                    if (!is_ancestor(p, proof_of_unit)) {
                        res = proof_of_unit;
                    }
                    else {
                        // -- failed to transform the proof, perhaps bad
                        // -- choice of the proof of unit
                        res = p;
                    }
                }
                else {
                    // -- no unit found to replace the hypothesis
                    res = p;
                }
            }

            else if (!dirty) {res = p;}

            else if (m.is_lemma(p)) {
                // lemma: reduce the premise; remove reduced consequences
                // from conclusion
                SASSERT(args.size() == 1);
                res = mk_lemma_core(args[0], m.get_fact(p));
                // -- re-compute hypsets
                compute_hypsets(res);
            }
            else if (m.is_unit_resolution(p)) {
                // unit: reduce untis; reduce the first premise; rebuild
                // unit resolution
                res = mk_unit_resolution_core(p, args);
                // -- re-compute hypsets
                compute_hypsets(res);
            }
            else {
                res = mk_proof_core(p, args);
                // -- re-compute hypsets
                compute_hypsets(res);
            }

            SASSERT(res);
            m_cache.insert(p, res);

            // bail out as soon as found a sub-proof of false
            if (!m_open_mark.is_marked(res) && m.has_fact(res) && m.is_false(m.get_fact(res)))
                return res;
        }
        UNREACHABLE();
        return nullptr;
    }

    proof* hypothesis_reducer::mk_lemma_core(proof* premise, expr *fact) {
        SASSERT(m.is_false(m.get_fact(premise)));
        SASSERT(m_active_hyps.contains(premise));

        proof_ptr_vector* active_hyps = m_active_hyps.find(premise);

        // if there is no active hypothesis return the premise
        if (!m_open_mark.is_marked(premise)) {
            // XXX just in case premise might go away
            m_pinned.push_back(premise);
            return premise;
        }

        // add some stability
        std::stable_sort(active_hyps->begin(), active_hyps->end(), ast_lt_proc());
        // otherwise, build a disjunction of the negated active hypotheses
        // and add a lemma proof step
        expr_ref_buffer args(m);
        for (auto hyp : *active_hyps) {
            expr *hyp_fact, *t;
            hyp_fact = m.get_fact(hyp);
            if (m.is_not(hyp_fact, t))
                args.push_back(t);
            else
                args.push_back(m.mk_not(hyp_fact));
        }

        expr_ref lemma(m);
        lemma = mk_or(m, args.size(), args.c_ptr());

        proof* res;
        res = m.mk_lemma(premise, lemma);
        m_pinned.push_back(res);
        return res;
    }

    proof* hypothesis_reducer::mk_unit_resolution_core(proof *ures,
                                                       ptr_buffer<proof>& args) {
        // if any literal is false, we don't need a unit resolution step
        // This can be the case due to some previous transformations
        for (unsigned i = 1, sz = args.size(); i < sz; ++i) {
            if (m.is_false(m.get_fact(args[i]))) {
                // XXX pin just in case
                m_pinned.push_back(args[i]);
                return args[i];
            }
        }

        proof* arg0 = args[0];
        app *fact0 = to_app(m.get_fact(arg0));


        ptr_buffer<proof> pf_args;
        ptr_buffer<expr> pf_fact;
        pf_args.push_back(arg0);

        // compute literals to be resolved
        ptr_buffer<expr> lits;

        // fact0 is a literal whenever the original resolution was a
        // binary resolution to an empty clause
        if (m.get_num_parents(ures) == 2 && m.is_false(m.get_fact(ures))) {
            lits.push_back(fact0);
        }
        // fact0 is a literal unless it is a dijsunction
        else if (!m.is_or(fact0)) {
            lits.push_back(fact0);
        }
        // fact0 is a literal only if it appears as a literal in the
        // original resolution
        else {
            lits.reset();
            app* ures_fact = to_app(m.get_fact(m.get_parent(ures, 0)));
            for (unsigned i = 0, sz = ures_fact->get_num_args(); i < sz; ++i) {
                if (ures_fact->get_arg(i) == fact0) {
                    lits.push_back(fact0);
                    break;
                }
            }
            if (lits.empty()) {
                lits.append(fact0->get_num_args(), fact0->get_args());
            }

        }

        // -- find all literals that are resolved on
        for (unsigned i = 0, sz = lits.size(); i < sz; ++i) {
            bool found = false;
            for (unsigned j = 1; j < args.size(); ++j) {
                if (m.is_complement(lits.get(i), m.get_fact(args[j]))) {
                    found = true;
                    pf_args.push_back(args[j]);
                    break;
                }
            }
            if (!found) {pf_fact.push_back(lits.get(i));}
        }

        // unit resolution got reduced to noop
        if (pf_args.size() == 1) {
            // XXX pin just in case
            m_pinned.push_back(arg0);

            return arg0;
        }

        // make unit resolution proof step
        // expr_ref tmp(m);
        // tmp = mk_or(m, pf_fact.size(), pf_fact.c_ptr());
        // proof* res = m.mk_unit_resolution(pf_args.size(), pf_args.c_ptr(), tmp);
        proof *res = m.mk_unit_resolution(pf_args.size(), pf_args.c_ptr());
        m_pinned.push_back(res);

        return res;
    }

    proof* hypothesis_reducer::mk_proof_core(proof* old, ptr_buffer<proof>& args) {
        // if any of the literals are false, we don't need a step
        for (unsigned i = 0; i < args.size(); ++i) {
            if (m.is_false(m.get_fact(args[i]))) {
                // XXX just in case
                m_pinned.push_back(args[i]);
                return args[i];
            }
        }

        // otherwise build step
        // BUG: I guess this doesn't work with quantifiers (since they are no apps)
        args.push_back(to_app(m.get_fact(old)));

        SASSERT(old->get_decl()->get_arity() == args.size());

        proof* res = m.mk_app(old->get_decl(), args.size(),
                              (expr * const*)args.c_ptr());
        m_pinned.push_back(res);
        return res;
    }

};