instance_var_bug.m:018: In instance declaration for `tc(V)':
instance_var_bug.m:018:   the first instance type `V' is a type variable; it
instance_var_bug.m:018:   should be a type constructor applied to zero or more
instance_var_bug.m:018:   type variables.
instance_var_bug.m:018:   (Every type in an instance declaration must consist
instance_var_bug.m:018:   of a type constructor applied to zero or more type
instance_var_bug.m:018:   variables as arguments.)