def _TriDict(tris):
    """tris is a list of triangles (a,b,c), CCW-oriented indices.
    Return dict mapping all edges in the triangles to the containing
    triangle list."""
    ans = dict()
    for i in range(0, len(tris)):
        a, b, c = t = tris[i]
        ans[a, b] = t
        ans[b, c] = t
        ans[c, a] = t
    return ans
