cx_and
lhs = expr_eval(expr->cx_and.left, fn, ctx);
rhs = expr_eval(expr->cx_and.right, fn, ctx);
cx->cx_and.left = left;
cx->cx_and.right = right;
cx->cx_and.left = NULL;
cx->cx_and.right = NULL;
condexpr_destroy(expr->cx_and.left);
condexpr_destroy(expr->cx_and.right);