dsl_arithmetic_5_functor_instance.h

template<typename A, typename M>
auto fmap(M map, expression_r<A> const& e)
{
   using B = decltype(map(std::declval<A>()));
   using Out = expression_r<B>;
   
   if (auto* o = get_as_add(e))
      return Out(add_op<B>(o->rands() | transformed(map)));
      
   if (auto* o = get_as_mul(e))
      return Out(mul_op<B>(o->rands() | transformed(map)));
   
   if (auto* i = get_as_cst(e)) return Out(*i);
   if (auto* v = get_as_var(e)) return Out(*v);
   throw_missing_pattern_matching_clause();
}