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 G being connected _Graph, v being Vertex of G holds v is non
  cut-vertex iff for G2 being removeVertex of G,v holds G2.numComponents() c= G
  .numComponents() by Lm19;
