Error found:
in module Main
at tests/purs/failing/ClassHeadNoVTA6b.purs:16:19 - 16:33 (line 16, column 19 - line 16, column 33)

  No type class instance was found for
                             
    Main.MultiCoveringSets a0
                           t3
                           c1
                           d2
                           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.partialOfABSet
      One of the following sets of type variables:
        b
        e, f

while checking that type forall (t70 :: Type) (t71 :: Type) (@a :: Type) (@b :: t70) (@c :: Type) (@d :: Type) (@e :: t71) (@f :: Type).
                           MultiCoveringSets @t70 @t71 a b c d e f => a                                                                 
                                                                      -> { c :: c                                                       
                                                                         , d :: d                                                       
                                                                         }                                                              
  is at least as general as type a0          
                                 -> { c :: c1
                                    , d :: d2
                                    }        
while checking that expression partialOfABSet
  has type a0          
           -> { c :: c1
              , d :: d2
              }        
in value declaration partialOfABSet'

where a0 is a rigid type variable
        bound at (line 16, column 19 - line 16, column 33)
      c1 is a rigid type variable
        bound at (line 16, column 19 - line 16, column 33)
      d2 is a rigid type variable
        bound at (line 16, column 19 - line 16, column 33)
      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.