This will create a rule that converts the expression $from to $to,
and then apply that rule to $expr. $from, $to, $expr are all
Math::Expr::Opp structures that should be Simplified to work ok.
The result is a array @res of Math::Expr::Opp objects which should
contain the results of applying the rule once on $expr in all
possible ways. They are all simplified, and duplicaions are removed.