code: function destroy(foPairs, index, collection, operand) {
    if (foPairs[index][collection].size === 1) {
      if (containsOne(foPairs[index])) {
        delete foPairs[index];
      } else {
        delete foPairs[index][collection];
      }
    }else {
      foPairs[index][collection].delete(operand);
    }
  }
docs: Performs a cascading removal of a field-operand pair
  
  @param foPairs
  @param index
  @param collection
  @param operand