Used by sorting routines only; not useful for matching (which is a much more interesting operation).
compare in interface Comparator
o1 - one tuple or subclass
o2 - another tuple or subclass
negative integer, zero or positive integer, depending on whether t1's relation label is less than, equal to, or greater
than t2's relation label. Ignores case of labels. If relations are equal, compares concept1's label. If still equal, tries
concept t2's label, then gives up.