  We introduce Composer's Assistant, a system for interactive human-computer
composition in the REAPER digital audio workstation. We consider the task of
multi-track MIDI infilling when arbitrary track-measures have been deleted from
a contiguous slice of measures from a MIDI file, and we train a T5-like model
to accomplish this task. Composer's Assistant consists of this model together
with scripts that enable interaction with the model in REAPER. We conduct
objective and subjective tests of our model. We release our complete system,
consisting of source code, pretrained models, and REAPER scripts. Our models
were trained only on permissively-licensed MIDI files.
