theorem Th19:
 [x,x1,x2,x3] in X implies x in proj1_4 X
proof
 assume [x,x1,x2,x3] in X;
  then [[x,x1],x2,x3] in X;
  then [x,x1] in proj1_3 X by Th13;
 hence thesis by Def12;
end;
