theorem Th26:
  for f be Function holds Ext(f,x,y)|X = Ext(f|X,x,y)
proof
  let f be Function;
  thus  Ext(f,x,y)|X = Ext(f,x,y)*(id X) by RELAT_1:65
  .=Ext(f*(id X),x,y) by Th25
  .=Ext(f|X,x,y) by RELAT_1:65;
end;
