[graded-base] Predicate missing polyshape constraints

[dorchard]

This works:

language GradedBase
data Bool = True | False
flib : Bool % 2 -> (Bool, Bool)
flib x = (case x of True -> True; False -> False, case  x of True -> True; False -> False)

But the predicate does not seem to be includes the poly shape conditions and this is worrying.... there is probably then an example which should fail and doesn't because of this.