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

theorem Th15:
  X (/\) Y c= X
proof
    let i be object such that
A1: i in I;
    let e be object;
    assume e in (X (/\) Y).i;
    then e in X.i /\ Y.i by A1,Def5;
    hence thesis by XBOOLE_0:def 4;
end;
