  Call a string-to-string transducer regular if it can be realised by one of
the following equivalent models: mso transductions, two-way deterministic
automata with output, and streaming transducers with registers. This paper
proposes to treat origin information as part of the semantics of a regular
string-to-string transducer. With such semantics, the model admits a
machine-independent characterisation, Angluin-style learning in polynomial
time, as well as effective characterisations of natural subclasses such as
one-way transducers or first-order definable transducers.
