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