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
  G.reachableDFrom(v) c= G.reachableFrom(v)
proof
  set RFD = G.reachableDFrom(v);
  let x be object;
  assume
A1: x in RFD;
  then reconsider x9=x as Vertex of G;
  ex W being directed Walk of G st W is_Walk_from v,x9 by A1,Def6;
  hence thesis by Def5;
end;
