theorem
  G.edgesBetween(X) c= G.edgesInOut(X)
proof
  let z be object;
  assume z in G.edgesBetween(X);
  then z in G.edgesInto(X) by XBOOLE_0:def 4;
  hence z in G.edgesInOut(X) by XBOOLE_0:def 3;
end;
