& package:constraints

class (p, q) => p & q
constraints Data.Constraint
due to the hack for the kind of (,) in the current version of GHC we can't actually make instances for (,) :: Constraint -> Constraint -> Constraint, but we can define an equivalent type, that converts back and forth to (,), and lets you hang instances.
(&&&) :: (a :- b) -> (a :- c) -> a :- (b, c)
constraints Data.Constraint
Constraint product 
trans weaken1 (f &&& g) = f
trans weaken2 (f &&& g) = g