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;
