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 Th43:
  G1 == G2 & G1 is acyclic implies G2 is acyclic
proof
  assume that
A1: G1 == G2 and
A2: G1 is acyclic;
  reconsider G19 = G1 as acyclic _Graph by A2;
  G2 is Subgraph of G19 by A1,GLIB_000:87;
  hence thesis;
end;
