This usually means that some list is constrained to be empty, and it is attempting to unify with something constrained to be non-empty. For instance, if a supertype constrains COMPS to be empty (COMPS < >), and another supertype (or the current type) is trying to constrain COMPS somehow (COMPS <[ LOCAL.CONT.HOOK.INDEX #ind ]> or COMPS.FIRST.LOCAL.CONT.HOOK.INDEX.PNG 3pl).

-- Main.trimblet - 2014-05-23