  Deterministic two-way transducers define the class of regular functions from
words to words. Alur and Cern\'y introduced an equivalent model of transducers
with registers called copyless streaming string transducers. In this paper, we
drop the "copyless" restriction on these machines and show that they are
equivalent to two-way transducers enhanced with the ability to drop marks,
named "marbles", on the input. We relate the maximal number of marbles used
with the amount of register copies performed by the streaming string
transducer. Finally, we show that the class membership problems associated with
these models are decidable. Our results can be interpreted in terms of program
optimization for simple recursive and iterative programs.
