Error found:
in module Main
at tests/purs/failing/ClassHeadNoVTA6a.purs:12:15 - 12:25 (line 12, column 15 - line 12, column 25)

  No type class instance was found for
                             
    Main.MultiCoveringSets t0
                           t1
                           t2
                           t3
                           t4
                           t5
                             
  The instance head contains unknown type variables.

  Note: The following type class members found in the expression require visible type applications to be unambiguous (e.g. tyClassMember @Int).
    Main.noneOfSets
      One of the following sets of type variables:
        a, b
        e, f

while checking that type forall (t82 :: Type) (t83 :: Type) (@a :: Type) (@b :: t82) (@c :: Type) (@d :: Type) (@e :: t83) (@f :: Type). MultiCoveringSets @t82 @t83 a b c d e f => Int
  is at least as general as type Int
while checking that expression noneOfSets
  has type Int
in value declaration noneOfSets'

where t0 is an unknown type
      t1 is an unknown type
      t2 is an unknown type
      t3 is an unknown type
      t4 is an unknown type
      t5 is an unknown type

See https://github.com/purescript/documentation/blob/master/errors/NoInstanceFound.md for more information,
or to contribute content related to this error.