diff --git a/src/library/expr_lt.h b/src/library/expr_lt.h
index cbd70b437..cc40af234 100644
--- a/src/library/expr_lt.h
+++ b/src/library/expr_lt.h
@@ -19,4 +19,5 @@ inline bool operator<(expr const & a, expr const & b)  { return is_lt(a, b, true
 inline bool operator>(expr const & a, expr const & b)  { return is_lt(b, a, true); }
 inline bool operator<=(expr const & a, expr const & b) { return !is_lt(b, a, true); }
 inline bool operator>=(expr const & a, expr const & b) { return !is_lt(a, b, true); }
+struct expr_quick_cmp { int operator()(expr const & e1, expr const & e2) const { return is_lt(e1, e2, true) ? -1 : (e1 == e2 ? 0 : 1); } };
 }