reserve G,G1,G2 for _Graph;
reserve e,x,y for set;
reserve v,v1,v2 for Vertex of G;
reserve W for Walk of G;

theorem
  for v1 being Vertex of G1, v2 being Vertex of G2 st G1 == G2 & v1 = v2
  holds G1.reachableFrom(v1) = G2.reachableFrom(v2) by Lm9;
