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

theorem
  Y does not overlap Z implies X (/\) Y does not overlap X (/\) Z
proof
  assume
A1: Y does not overlap Z;
  X (/\) Y c= Y & X (/\) Z c= Z by Th15;
  hence thesis by A1,Th101;
end;
