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

theorem Th14:
  X c= X (\/) Y
proof
    let i be object such that
A1: i in I;
    let e be object;
    assume e in X.i;
    then e in X.i \/ Y.i by XBOOLE_0:def 3;
    hence thesis by A1,Def4;
end;
