pretty_print_infix_good.cpp

std::string print_op_infix(op<add_tag, std::pair<std::string, expression const*>> const& e)
{
   auto fst = [](auto const& e) { return e.first; }; 
   return boost::algorithm::join(e.rands() | transformed(fst), " + ");
}

std::string print_op_infix(op<mul_tag, std::pair<std::string, expression const*>> const& e)
{
   auto wrap_addition = [](auto const& sub_expr) {
      if (get_as_add(sub_expr.second->get()))
         return with_parens(sub_expr.first);
      return sub_expr.first;
   };
   return boost::algorithm::join(e.rands() | transformed(wrap_addition), " * ");
}

std::string print_infix(expression_r<std::pair<std::string, expression const*>> const& e)
{
   if (auto* o = get_as_add(e)) return print_op_infix(*o);
   if (auto* o = get_as_mul(e)) return print_op_infix(*o);
   if (auto* i = get_as_cst(e)) return std::to_string(*i);
   if (auto* v = get_as_var(e)) return *v;
   throw_missing_pattern_matching_clause();
}