mergeOrderings(orderings, seen=None)
Merge multiple orderings so that within-ordering order is preserved
Orderings are constrained in such a way that if an object appears in two or more orderings, then the suffix that begins with the object must be in both orderings.
For example:
>>> _mergeOrderings([ ... ['x', 'y', 'z'], ... ['q', 'z'], ... [1, 3, 5], ... ['z'] ... ]) ['x', 'y', 'q', 1, 3, 5, 'z']