'Given two points, ``pt1`` and ``pt2``, and the welding edge
``edge``, chooses one of the two points to place the opposing
vertex ``obj`` of the triangle.  If neither of this points
fits, returns ``None``.'

@staticmethod
def _choose_target_cell(pt1, pt2, edge, obj, skeleton, grid):

    pt1_empty = DiagramGrid._empty_point(pt1, grid)
    pt2_empty = DiagramGrid._empty_point(pt2, grid)
    if (pt1_empty and pt2_empty):
        A = grid[edge[0]]
        B = grid[edge[1]]
        if skeleton.get(frozenset([A, obj])):
            return pt1
        else:
            return pt2
    if pt1_empty:
        return pt1
    elif pt2_empty:
        return pt2
    else:
        return None
