code: def sign(pvt, id, txn)
        raise 'pvt must be of type Key' unless pvt.is_a?(Key)
        raise 'id must be of type Id' unless id.is_a?(Id)
        raise 'txn must be of type Txn' unless txn.is_a?(Txn)
        pvt.sign(body(id, txn))
      end
docs: Sign the trasnsaction and return the signature.
   +pvt+:: Private RSA key
   +id+:: Paying wallet ID
   +txn+:: The transaction