Update ADR2 to address type ascriptions for empty collections (#256)
Update ADR2 to address type ascriptions for empty collections (#256)

* Update ADR2 to support better type errors

Broken out of #244

This updates and ADR2 in anticipation of the addition
of the empty collection typing operators `EmptySet` and
`EmptySeq`.
konnov/apalache,konnov/apalache