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 e,x,y being object holds
  x in G.reachableFrom(v1) & e Joins x,y,G implies y in G.reachableFrom(
  v1) by Lm2;
