Parse vertex indices in '/' separated form (like 'i/j/k', 'i//k' ...).

def _parse_vertex_tuple(s):
  """Parse vertex indices in '/' separated form (like 'i/j/k', 'i//k' ...)."""
  vt = [0, 0, 0]
  for i, c in enumerate(s.split('/')):
    if c:
      vt[i] = int(c)
  return tuple(vt)