This is the development site of the LiquidHaskell formal verification tool. If you're a LiquidHaskell user (or just curious), you probably want to go to the documentation website instead. LiquidHaskell (LH) refines Haskell's types with logical predicates that let you enforce important properties at compile time. LH warns you that head is not total as it is missing the case for [] and checks that it is total on NonEmpty lists. The input contract propagates to uses of head which are verified...