assume
if (assume(comparison)) {
if (!assume(success))
int assume(struct expression *expr);
if (!assume(compare_expression(expr, separate_zero ? '>' : SPECIAL_GTE, zero_expr())))
undo = assume(compare_expression(expr, SPECIAL_EQUAL, zero_expr()));
undo = assume(compare_expression(expr, '<', zero_expr()));
if (!assume(comparison))