Support arbitrarily sized multi-inputs

[t3kt]

This could potentially be done by having a merge operator which combines some fixed max number of definitions (e.g. 4), but marks the primary for each input in the resulting table. These could then be chained and merged and then fed into a combine operator that would look for those primary definitions.

This could be used to address #101, at least for multi-input cases. It wouldn't handle cases that don't use multi-input though.