New tactic: "proc case"

[mbbarbosa]

New use case: would be super useful if this also works when we have

lv <- (e1,...,en)

Unfolded as

lv.1 <- e1 ... lv.n <- en

[fdupress]

This is currently not valid EC, so we'd need to figure out a meaning for

lv.`1 <- e

First.

[strub]

This raises the question of having more general left-values.