code: 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
docs: Returns TRUE if the wallet contains a payment received with the specified
   ID, which was sent by the specified beneficiary.