message: not all vars missing in a model are undefs
diff: #include <map>
  #include <set>
  #include <sstream>
  +#include <string_view>
  using namespace IR;
  using namespace smt;
  @@ -59,6 +60,16 @@ static void print_varval(ostream &s, State &st, const Model &m,
  // undef variables may not have a model since each read uses a copy
  // TODO: add intervals of possible values for ints at least?
  if (!partial.isConst()) {
  + // some functions / vars may not have an interpretation because it's not
  + // needed, not because it's undef
  + bool found_undef = false;
  + for (auto &var : partial.vars()) {
  + ostringstream ss;
  + ss << var;
  + auto name = ss.str();
  + found_undef |= string_view(name).substr(0, 6) == "undef_";
  + }
  + if (found_undef)
  s << "\t[based on undef value]";
  }
  }
  