theorem
 for e,x,y being object holds
  G1 == G2 implies (e Joins x,y,G1 implies e Joins x,y,G2) & (e
DJoins x,y,G1 implies e DJoins x,y,G2) & (e SJoins X,Y,G1 implies e SJoins X,Y,
  G2) & (e DSJoins X,Y,G1 implies e DSJoins X,Y,G2);
