mirror of
https://github.com/Z3Prover/z3
synced 2025-04-23 17:15:31 +00:00
Dev (#63)
* introduce int_solver.h Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * add int_solver class Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * track which var is an integer Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * add queries for integrality of vars Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * resurrect lp_tst in its own director lp Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * add file Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * add_constraint has got a body Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * fix add_constraint and substitute_terms_in_linear_expression Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * after merge with Z3Prover Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * adding stub check_int_feasibility() Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * Dev (#50) * initial skeletons for nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * initial skeletons for nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * small fix in lar_solver.cpp Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * adding some content to the new check_int_feasibility() Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * Dev (#51) * initial skeletons for nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * initial skeletons for nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding more nlsat Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * nlsat integration Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding constraints Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * add missing initialization Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * test Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * Dev (#53) * change in a comment Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * Disabled debug output * removing FOCI2 interface from interp * remove foci reference from cmakelist.txt Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * initial skeletons for nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * initial skeletons for nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding more nlsat Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * nlsat integration Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding constraints Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * add missing initialization Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding nra Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * debugging nra Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * updates to nra_solver integration to call it directly from theory_lra instead of over lar_solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * n/a Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * integrate nlsat Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * tidy Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * preserve is_int flag Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * remove a debug printout Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * Dev (#54) * change in a comment Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * Disabled debug output * removing FOCI2 interface from interp * remove foci reference from cmakelist.txt Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * initial skeletons for nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * initial skeletons for nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding more nlsat Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * nlsat integration Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding constraints Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * add missing initialization Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding nra solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * adding nra Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * debugging nra Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * updates to nra_solver integration to call it directly from theory_lra instead of over lar_solver Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * n/a Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * integrate nlsat Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * tidy Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * use integer test from lra solver, updated it to work on term variables Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * fix equality check in assume-eq Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * fix model_is_int_feasible Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * untested gcd_test() Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * call fill_explanation_from_fixed_columns() Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * add the call to pivot_fixed_vars_from_basis() to int_solver.cpp::check() Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * port more of theory_arith_int.h Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * use statistics of lar_solver by theory_lra.cpp Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * port more code to int_solver.cpp Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * add an assert Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * more int porting Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * fix a bug in pivot_fixed_vars_from_basis Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * small change Signed-off-by: Lev Nachmanson <levnach@hotmail.com> * implement find_inf_int_base_column() Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * catch unregistered vars in add_var_bound Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * add a file Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * compile for vs2012 Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * fix asserts in add_var_bound Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * fix the lp_solver init when workig on an mps file Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * towards int_solver::check() Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * change in int_solver::check() signature Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * add handlers for lia moves Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * spacing Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * return branch from int_solver::check() Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * add a stub for mk_gomory_cut Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * Dev (#59) * add handlers for lia moves Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * spacing Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * loops Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * Dev (#60) * add handlers for lia moves Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * spacing Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * loops Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * loops Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * Dev (#61) * add handlers for lia moves Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * spacing Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * loops Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * loops Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * more TRACE(arith_int) Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * fix the build Signed-off-by: Lev Nachmanson <levnach@microsoft.com> * loops Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * Dev (#62) * add handlers for lia moves Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * spacing Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * loops Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * loops Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * loops Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com> * build fix Signed-off-by: Lev Nachmanson <levnach@microsoft.com>
This commit is contained in:
Nikolaj Bjorner
Lev Nachmanson
parent
a28a8304b7
commit
4c23527974
5 changed files with 291 additions and 57 deletions
|
|
@ -101,6 +101,186 @@ int int_solver::find_inf_int_boxed_base_column_with_smallest_range() {
|
|||
|
||||
}
|
||||
|
||||
bool int_solver::mk_gomory_cut(unsigned row_index, explanation & ex) {
|
||||
lean_assert(false);
|
||||
return true;
|
||||
/*
|
||||
const auto & row = m_lar_solver->A_r().m_rows[row_index];
|
||||
// The following assertion is wrong. It may be violated in mixed-integer problems.
|
||||
// SASSERT(!all_coeff_int(r));
|
||||
theory_var x_i = r.get_base_var();
|
||||
|
||||
SASSERT(is_int(x_i));
|
||||
// The following assertion is wrong. It may be violated in mixed-real-interger problems.
|
||||
// The check is_gomory_cut_target will discard rows where any variable contains infinitesimals.
|
||||
// SASSERT(m_value[x_i].is_rational()); // infinitesimals are not used for integer variables
|
||||
SASSERT(!m_value[x_i].is_int()); // the base variable is not assigned to an integer value.
|
||||
|
||||
if (constrain_free_vars(r) || !is_gomory_cut_target(r)) {
|
||||
TRACE("gomory_cut", tout << "failed to apply gomory cut:\n";
|
||||
tout << "constrain_free_vars(r): " << constrain_free_vars(r) << "\n";);
|
||||
return false;
|
||||
}
|
||||
|
||||
TRACE("gomory_cut", tout << "applying cut at:\n"; display_row_info(tout, r););
|
||||
|
||||
antecedents ante(*this);
|
||||
|
||||
m_stats.m_gomory_cuts++;
|
||||
|
||||
// gomory will be pol >= k
|
||||
numeral k(1);
|
||||
buffer<row_entry> pol;
|
||||
|
||||
numeral f_0 = Ext::fractional_part(m_value[x_i]);
|
||||
numeral one_minus_f_0 = numeral(1) - f_0;
|
||||
SASSERT(!f_0.is_zero());
|
||||
SASSERT(!one_minus_f_0.is_zero());
|
||||
|
||||
numeral lcm_den(1);
|
||||
unsigned num_ints = 0;
|
||||
|
||||
typename vector<row_entry>::const_iterator it = r.begin_entries();
|
||||
typename vector<row_entry>::const_iterator end = r.end_entries();
|
||||
for (; it != end; ++it) {
|
||||
if (!it->is_dead() && it->m_var != x_i) {
|
||||
theory_var x_j = it->m_var;
|
||||
numeral a_ij = it->m_coeff;
|
||||
a_ij.neg(); // make the used format compatible with the format used in: Integrating Simplex with DPLL(T)
|
||||
if (is_real(x_j)) {
|
||||
numeral new_a_ij;
|
||||
if (at_lower(x_j)) {
|
||||
if (a_ij.is_pos()) {
|
||||
new_a_ij = a_ij / one_minus_f_0;
|
||||
}
|
||||
else {
|
||||
new_a_ij = a_ij / f_0;
|
||||
new_a_ij.neg();
|
||||
}
|
||||
k.addmul(new_a_ij, lower_bound(x_j).get_rational());
|
||||
lower(x_j)->push_justification(ante, new_a_ij, coeffs_enabled());
|
||||
}
|
||||
else {
|
||||
SASSERT(at_upper(x_j));
|
||||
if (a_ij.is_pos()) {
|
||||
new_a_ij = a_ij / f_0;
|
||||
new_a_ij.neg(); // the upper terms are inverted.
|
||||
}
|
||||
else {
|
||||
new_a_ij = a_ij / one_minus_f_0;
|
||||
}
|
||||
k.addmul(new_a_ij, upper_bound(x_j).get_rational());
|
||||
upper(x_j)->push_justification(ante, new_a_ij, coeffs_enabled());
|
||||
}
|
||||
TRACE("gomory_cut_detail", tout << a_ij << "*v" << x_j << " k: " << k << "\n";);
|
||||
pol.push_back(row_entry(new_a_ij, x_j));
|
||||
}
|
||||
else {
|
||||
++num_ints;
|
||||
SASSERT(is_int(x_j));
|
||||
numeral f_j = Ext::fractional_part(a_ij);
|
||||
TRACE("gomory_cut_detail",
|
||||
tout << a_ij << "*v" << x_j << "\n";
|
||||
tout << "fractional_part: " << Ext::fractional_part(a_ij) << "\n";
|
||||
tout << "f_j: " << f_j << "\n";
|
||||
tout << "f_0: " << f_0 << "\n";
|
||||
tout << "one_minus_f_0: " << one_minus_f_0 << "\n";);
|
||||
if (!f_j.is_zero()) {
|
||||
numeral new_a_ij;
|
||||
if (at_lower(x_j)) {
|
||||
if (f_j <= one_minus_f_0) {
|
||||
new_a_ij = f_j / one_minus_f_0;
|
||||
}
|
||||
else {
|
||||
new_a_ij = (numeral(1) - f_j) / f_0;
|
||||
}
|
||||
k.addmul(new_a_ij, lower_bound(x_j).get_rational());
|
||||
lower(x_j)->push_justification(ante, new_a_ij, coeffs_enabled());
|
||||
}
|
||||
else {
|
||||
SASSERT(at_upper(x_j));
|
||||
if (f_j <= f_0) {
|
||||
new_a_ij = f_j / f_0;
|
||||
}
|
||||
else {
|
||||
new_a_ij = (numeral(1) - f_j) / one_minus_f_0;
|
||||
}
|
||||
new_a_ij.neg(); // the upper terms are inverted
|
||||
k.addmul(new_a_ij, upper_bound(x_j).get_rational());
|
||||
upper(x_j)->push_justification(ante, new_a_ij, coeffs_enabled());
|
||||
}
|
||||
TRACE("gomory_cut_detail", tout << "new_a_ij: " << new_a_ij << " k: " << k << "\n";);
|
||||
pol.push_back(row_entry(new_a_ij, x_j));
|
||||
lcm_den = lcm(lcm_den, denominator(new_a_ij));
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
CTRACE("empty_pol", pol.empty(), display_row_info(tout, r););
|
||||
|
||||
expr_ref bound(get_manager());
|
||||
if (pol.empty()) {
|
||||
SASSERT(k.is_pos());
|
||||
// conflict 0 >= k where k is positive
|
||||
set_conflict(ante, ante, "gomory-cut");
|
||||
return true;
|
||||
}
|
||||
else if (pol.size() == 1) {
|
||||
theory_var v = pol[0].m_var;
|
||||
k /= pol[0].m_coeff;
|
||||
bool is_lower = pol[0].m_coeff.is_pos();
|
||||
if (is_int(v) && !k.is_int()) {
|
||||
k = is_lower?ceil(k):floor(k);
|
||||
}
|
||||
rational _k = k.to_rational();
|
||||
if (is_lower)
|
||||
bound = m_util.mk_ge(get_enode(v)->get_owner(), m_util.mk_numeral(_k, is_int(v)));
|
||||
else
|
||||
bound = m_util.mk_le(get_enode(v)->get_owner(), m_util.mk_numeral(_k, is_int(v)));
|
||||
}
|
||||
else {
|
||||
if (num_ints > 0) {
|
||||
lcm_den = lcm(lcm_den, denominator(k));
|
||||
TRACE("gomory_cut_detail", tout << "k: " << k << " lcm_den: " << lcm_den << "\n";
|
||||
for (unsigned i = 0; i < pol.size(); i++) {
|
||||
tout << pol[i].m_coeff << " " << pol[i].m_var << "\n";
|
||||
}
|
||||
tout << "k: " << k << "\n";);
|
||||
SASSERT(lcm_den.is_pos());
|
||||
if (!lcm_den.is_one()) {
|
||||
// normalize coefficients of integer parameters to be integers.
|
||||
unsigned n = pol.size();
|
||||
for (unsigned i = 0; i < n; i++) {
|
||||
pol[i].m_coeff *= lcm_den;
|
||||
SASSERT(!is_int(pol[i].m_var) || pol[i].m_coeff.is_int());
|
||||
}
|
||||
k *= lcm_den;
|
||||
}
|
||||
TRACE("gomory_cut_detail", tout << "after *lcm\n";
|
||||
for (unsigned i = 0; i < pol.size(); i++) {
|
||||
tout << pol[i].m_coeff << " * v" << pol[i].m_var << "\n";
|
||||
}
|
||||
tout << "k: " << k << "\n";);
|
||||
}
|
||||
mk_polynomial_ge(pol.size(), pol.c_ptr(), k.to_rational(), bound);
|
||||
}
|
||||
TRACE("gomory_cut", tout << "new cut:\n" << bound << "\n"; ante.display(tout););
|
||||
literal l = null_literal;
|
||||
context & ctx = get_context();
|
||||
ctx.internalize(bound, true);
|
||||
l = ctx.get_literal(bound);
|
||||
ctx.mark_as_relevant(l);
|
||||
dump_lemmas(l, ante);
|
||||
ctx.assign(l, ctx.mk_justification(
|
||||
gomory_cut_justification(
|
||||
get_id(), ctx.get_region(),
|
||||
ante.lits().size(), ante.lits().c_ptr(),
|
||||
ante.eqs().size(), ante.eqs().c_ptr(), ante, l)));
|
||||
return true;
|
||||
*/
|
||||
|
||||
}
|
||||
lia_move int_solver::check(lar_term& t, mpq& k, explanation& ex) {
|
||||
lean_assert(is_feasible());
|
||||
init_inf_int_set();
|
||||
|
|
@ -129,32 +309,35 @@ lia_move int_solver::check(lar_term& t, mpq& k, explanation& ex) {
|
|||
|
||||
if ((++m_branch_cut_counter) % settings().m_int_branch_cut_threshold == 0) {
|
||||
move_non_base_vars_to_bounds();
|
||||
/*
|
||||
if (!make_feasible()) {
|
||||
TRACE("arith_int", tout << "failed to move variables to bounds.\n";);
|
||||
failed();
|
||||
return FC_CONTINUE;
|
||||
lp_status st = m_lar_solver->find_feasible_solution();
|
||||
if (st != lp_status::FEASIBLE && st != lp_status::OPTIMAL) {
|
||||
return lia_move::give_up;
|
||||
}
|
||||
int int_var = find_inf_int_base_var();
|
||||
if (int_var != null_int) {
|
||||
TRACE("arith_int", tout << "v" << int_var << " does not have an integer assignment: " << get_value(int_var) << "\n";);
|
||||
SASSERT(is_base(int_var));
|
||||
row const & r = m_rows[get_var_row(int_var)];
|
||||
if (!mk_gomory_cut(r)) {
|
||||
int j = find_inf_int_base_column();
|
||||
if (j != -1) {
|
||||
TRACE("arith_int", tout << "j = " << j << " does not have an integer assignment: " << get_value(j) << "\n";);
|
||||
unsigned row_index = m_lar_solver->m_mpq_lar_core_solver.m_r_heading[j];
|
||||
if (!mk_gomory_cut(row_index, ex)) {
|
||||
return lia_move::give_up;
|
||||
// silent failure
|
||||
}
|
||||
return FC_CONTINUE;
|
||||
}*/
|
||||
return lia_move::cut;
|
||||
}
|
||||
}
|
||||
else {
|
||||
int j = find_inf_int_base_column();
|
||||
/*
|
||||
if (j != -1) {
|
||||
TRACE("arith_int", tout << "v" << j << " does not have an integer assignment: " << get_value(j) << "\n";);
|
||||
// apply branching
|
||||
branch_infeasible_int_var(int_var);
|
||||
return false;
|
||||
}*/
|
||||
TRACE("arith_int", tout << "j" << j << " does not have an integer assignment: " << get_value(j) << "\n";);
|
||||
|
||||
lean_assert(t.is_empty());
|
||||
t.add_to_map(j, mpq(1));
|
||||
k = floor(get_value(j));
|
||||
TRACE("arith_int", tout << "branching v" << j << " = " << get_value(j) << "\n";
|
||||
display_column(tout, j);
|
||||
tout << "k = " << k << std::endl;
|
||||
);
|
||||
return lia_move::branch;
|
||||
}
|
||||
}
|
||||
// return true;
|
||||
return lia_move::give_up;
|
||||
|
|
@ -259,7 +442,6 @@ mpq get_denominators_lcm(iterator_on_row<mpq> &it) {
|
|||
|
||||
bool int_solver::gcd_test_for_row(static_matrix<mpq, numeric_pair<mpq>> & A, unsigned i, explanation & ex) {
|
||||
iterator_on_row<mpq> it(A.m_rows[i]);
|
||||
std::cout << "gcd_test_for_row(" << i << ")\n";
|
||||
mpq lcm_den = get_denominators_lcm(it);
|
||||
mpq consts(0);
|
||||
mpq gcds(0);
|
||||
|
|
@ -350,8 +532,6 @@ bool int_solver::ext_gcd_test(iterator_on_row<mpq> & it,
|
|||
mpq const & least_coeff,
|
||||
mpq const & lcm_den,
|
||||
mpq const & consts, explanation& ex) {
|
||||
|
||||
std::cout << "calling ext_gcd_test" << std::endl;
|
||||
mpq gcds(0);
|
||||
mpq l(consts);
|
||||
mpq u(consts);
|
||||
|
|
|
|||
|
|
@ -96,5 +96,7 @@ private:
|
|||
int find_inf_int_boxed_base_column_with_smallest_range();
|
||||
lp_settings& settings();
|
||||
void move_non_base_vars_to_bounds();
|
||||
void branch_infeasible_int_var(unsigned);
|
||||
bool mk_gomory_cut(unsigned row_index, explanation & ex);
|
||||
};
|
||||
}
|
||||
|
|
|
|||
|
|
@ -21,6 +21,10 @@ struct lar_term {
|
|||
}
|
||||
}
|
||||
|
||||
bool is_empty() const {
|
||||
return m_coeffs.size() == 0 && is_zero(m_v);
|
||||
}
|
||||
|
||||
unsigned size() const { return static_cast<unsigned>(m_coeffs.size()); }
|
||||
|
||||
const std::unordered_map<unsigned, mpq> & coeffs() const {
|
||||
|
|
|
|||
Loading…
Add table
Add a link
Reference in a new issue