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

theorem Th59:
  X (\) EmptyMS I = X
proof
    let i be object;
    assume i in I;
    hence (X (\) EmptyMS I).i = X.i \ EmptyMS I.i by Def6
      .= X.i \ {}
      .= X.i;
end;
