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
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
Returns TRUE if the wallet contains a payment sent with the specified
 ID, which was sent to the specified beneficiary.