theorem Th25:
  X1 |-| X2 implies X1 \/ Y |-| X2 \/ Y
proof
  assume X1 |-| X2;
  then Cn(X1) = Cn(X2) by Th20;
  then Cn(X1 \/ Y) = Cn(Cn(X2) \/ Cn(Y)) by Th22
    .= Cn(X2 \/ Y) by Th22;
  hence thesis by Th20;
end;
