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 is_DTree_rooted_at x implies x is Vertex of G
proof
  set v = the Vertex of G;
  assume G is_DTree_rooted_at x;
  then ex W being DWalk of G st W is_Walk_from x,v;
  hence thesis by GLIB_001:18;
end;
