
theorem Th15:
  for G1, G2 being _Graph, F being PGraphMapping of G1, G2
  holds F is semi-continuous iff
    for e,v,w being object st e in dom F_E & v in dom F_V & w in dom F_V
    holds e Joins v,w,G1 iff F_E.e Joins F_V.v, F_V.w, G2 by Th4;
