#include "bnf.h"

std::map<std::string, std::set<std::string>> Reverse(BNF bnf){
 std::map<std::string, std::set<std::string>> result;

 for (const auto& [from, to] : bnf) {
  for (const auto& list : to) {
   for (const auto& element : list) {
    auto i{result.find(element)};
    if (i != result.end()) // already present
     i->second.insert(from);
    else // new element
     result.emplace(element, std::set{from});
   }
  }
 }

 return result;
}