'For a given triangle always picks the same root edge.  The
root edge is the edge that will be placed first on the grid.'

@staticmethod
def _pick_root_edge(tri, skeleton):

    candidates = [sorted(e, key=default_sort_key) for e in tri if skeleton[e]]
    sorted_candidates = sorted(candidates, key=default_sort_key)
    return tuple(sorted(sorted_candidates[0], key=default_sort_key))
