T16474.hs:6:1:
    Quantified predicate must have a class or type variable head:
       forall a. A t a => A t [a]
    In the quantified constraint ‘forall a. A t a => A t [a]’
     In the context: forall a. A t a => A t [a]
     While checking the super-classes of class ‘B’
     In the class declaration for ‘B’