Adding a MIDI only plugin at a point where there is no
MIDI data, and/or additional audio signals results in an
'Impossible' match.
Those are usually resolved by trying to replicate the plugin,
and the fallback is to "Replicate 1 time".
While this is effectively equivalent with ExactMatch (use
1 instance), it is semantically different: Audio sources
will be ignored and if there is no MIDI signal, the MIDI
input remains unconnected.
It is the opposite to "Hide" (plugin has more inputs
of a given type, which can be fed by silence), since it has
fewer inputs of a given type signals are "dropped".
Strictly speaking we should special case this "Replicate 1 time"
case to "Drop" [sic]. which only assert(!reconfigurable_io()).
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