fix #6343

The bug was that axiom generation was not enabled on last_index, so no axioms got created to constrain last-index.
With default settings the solver is now very slow on this example. It is related to that the smallest size of a satisfying assignment is above 24. Pending a good heuristic to find initial seeds and increments for iterative deepening, I am adding another parameter smt.seq.min_unfolding that when set to 30 helps for this example.

src/ast/rewriter/seq_axioms.cpp

@@ -509,6 +509,8 @@ namespace seq {
     }
 
     /**
+       i = last_indexof(t, s):
+
        !contains(t, s) => i = -1   
        |t| = 0 => |s| = 0 or i = -1
        |t| = 0 & |s| = 0 => i = 0

src/smt/params/smt_params_helper.pyg

@@ -107,6 +107,7 @@ def_module_params(module_name='smt',
                           ('seq.split_w_len', BOOL, True, 'enable splitting guided by length constraints'),
                           ('seq.validate', BOOL, False, 'enable self-validation of theory axioms created by seq theory'),
                           ('seq.max_unfolding', UINT, 1000000000, 'maximal unfolding depth for checking string equations and regular expressions'),
+                          ('seq.min_unfolding', UINT, 1, 'initial bound for strings whose lengths are bounded by iterative deepening. Set this to a higher value if there are only models with larger string lengths'),
                           ('str.strong_arrangements', BOOL, True, 'assert equivalences instead of implications when generating string arrangement axioms'),
                           ('str.aggressive_length_testing', BOOL, False, 'prioritize testing concrete length values over generating more options'),
                           ('str.aggressive_value_testing', BOOL, False, 'prioritize testing concrete string constant values over generating more options'),

src/smt/params/theory_seq_params.cpp

@@ -22,4 +22,5 @@ void theory_seq_params::updt_params(params_ref const & _p) {
     m_split_w_len = p.seq_split_w_len();
     m_seq_validate = p.seq_validate();
     m_seq_max_unfolding = p.seq_max_unfolding();
+    m_seq_min_unfolding = p.seq_min_unfolding();
 }

src/smt/params/theory_seq_params.h

@@ -25,6 +25,7 @@ struct theory_seq_params {
     bool m_split_w_len = false;
     bool m_seq_validate = false;
     unsigned m_seq_max_unfolding = UINT_MAX/4;
+    unsigned m_seq_min_unfolding = 1;
 
     theory_seq_params(params_ref const & p = params_ref()) {
         updt_params(p);

src/smt/theory_seq.cpp

@@ -304,6 +304,7 @@ void theory_seq::init() {
     m_ax.add_axiom5 = add_ax;
     m_ax.mk_eq_empty2 = mk_eq_emp;
     m_arith_value.init(&ctx);
+    m_max_unfolding_depth = ctx.get_fparams().m_seq_min_unfolding;
 }
 
 #define TRACEFIN(s) { TRACE("seq", tout << ">>" << s << "\n";); IF_VERBOSE(20, verbose_stream() << s << "\n"); }
@@ -3256,6 +3257,7 @@ void theory_seq::relevant_eh(app* n) {
         m_util.str.is_from_code(n) ||
         m_util.str.is_to_code(n) ||
         m_util.str.is_unit(n) ||
+        m_util.str.is_last_index(n) ||
         m_util.str.is_length(n) || 
         /* m_util.str.is_replace_all(n) || uncomment to enable axiomatization */
         m_util.str.is_le(n)) {