reserve i,j,e,u for object;
reserve I for set; 
reserve x,X,Y,Z,V for ManySortedSet of I;

theorem Th56:
  X (\) Y c= X
proof
  now
    let i be object such that
A1: i in I;
    X.i \ Y.i c= X.i;
    hence (X (\) Y).i c= X.i by A1,Def6;
  end;
  hence thesis;
end;
