Logtk.IndexInterfaces for indexing terms (for unification, matching) and clauses (for subsumption)
type term = Term.ttype subst = Subst.tA leaf maps terms to a set of elements
module type LEAF = Index_intf.LEAFmodule type TERM_IDX = Index_intf.TERM_IDXmodule type CLAUSE = Index_intf.CLAUSEA subsumption index (non perfect!)
module type SUBSUMPTION_IDX = Index_intf.SUBSUMPTION_IDXmodule type EQUATION = Index_intf.EQUATIONmodule type UNIT_IDX = Index_intf.UNIT_IDX