+/** Cast the relational into a boolean, mainly for evaluation within an
+ * if-statement. Note that (a<b) == false does not imply (a>=b) == true in
+ * the general symbolic case. A false result means the comparison is either
+ * false or undecidable (except of course for !=, where true means either
+ * unequal or undecidable). */
+relational::operator bool() const
+{
+ const ex df = lh-rh;
+ if (!is_ex_exactly_of_type(df,numeric))
+ // cannot decide on non-numerical results
+ return o==not_equal ? true : false;
+
+ switch (o) {
+ case equal:
+ return ex_to<numeric>(df).is_zero();
+ case not_equal:
+ return !ex_to<numeric>(df).is_zero();
+ case less:
+ return ex_to<numeric>(df)<_num0;
+ case less_or_equal:
+ return ex_to<numeric>(df)<=_num0;
+ case greater:
+ return ex_to<numeric>(df)>_num0;
+ case greater_or_equal:
+ return ex_to<numeric>(df)>=_num0;
+ default:
+ throw(std::logic_error("invalid relational operator"));
+ }
+}