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
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
Sign the trasnsaction and return the signature.
 +pvt+:: Private RSA key
 +id+:: Paying wallet ID
 +txn+:: The transaction