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

theorem Th20:
  X c= Y & Z c= V implies X (\/) Z c= Y (\/) V
proof
  assume that
A1: X c= Y and
A2: Z c= V;
  V c= Y (\/) V by Th14;
  then
A3: Z c= Y (\/) V by A2,Th13;
  Y c= Y (\/) V by Th14;
  then X c= Y (\/) V by A1,Th13;
  hence thesis by A3,Th16;
end;
