Use `ε` for CMRA unit
For obsolete reasons, that no longer seem to apply, we used the Empty type class for the unit of a CMRA, which resulted in using the notation ∅. Using ε is more consistent with what we do on paper.
For obsolete reasons, that no longer seem to apply, we used the Empty type class for the unit of a CMRA, which resulted in using the notation ∅. Using ε is more consistent with what we do on paper.