theorem Th21:
 [x1,x,x2,x3] in X implies x in proj2_4 X
proof
 assume [x1,x,x2,x3] in X;
  then [[x1,x],x2,x3] in X;
  then [x1,x] in proj1_3 X by Th13;
 hence thesis by Def13;
end;
