code: def includes_negative?(id, bnf = nil)
        raise 'The txn ID has to be of type Integer' unless id.is_a?(Integer)
        !txns.find { |t| t.id == id && (bnf.nil? || t.bnf == bnf) && t.amount.negative? }.nil?
      end
docs: Returns TRUE if the wallet contains a payment sent with the specified
   ID, which was sent to the specified beneficiary.