polymorphic_unification.m:023: In clause for `p(in, ((list.list_skel) >>
polymorphic_unification.m:023:   dead))':
polymorphic_unification.m:023:   in polymorphically-typed unification:
polymorphic_unification.m:023:   mode error: variable `X' has instantiatedness
polymorphic_unification.m:023:     named inst list.list_skel
polymorphic_unification.m:023:     which expands to
polymorphic_unification.m:023:       named inst list.list_skel(free)
polymorphic_unification.m:023:       which expands to
polymorphic_unification.m:023:         bound(
polymorphic_unification.m:023:           []
polymorphic_unification.m:023:         ;
polymorphic_unification.m:023:           '[|]'(
polymorphic_unification.m:023:             free,
polymorphic_unification.m:023:             named inst list.list_skel(free)
polymorphic_unification.m:023:           )
polymorphic_unification.m:023:         ),
polymorphic_unification.m:023:   expected instantiatedness was `ground' or
polymorphic_unification.m:023:   `any'.
polymorphic_unification.m:023:   When unifying two variables whose type will
polymorphic_unification.m:023:   not be known until runtime, the variables must
polymorphic_unification.m:023:   both be ground (or have inst `any').
polymorphic_unification.m:023:   Unifications of polymorphically-typed
polymorphic_unification.m:023:   variables with partially instantiated modes
polymorphic_unification.m:023:   are not allowed.