reserve x,x1,x2,x3,x4,y,y1,y2,y3,y4,z,z1,z2,z2,z4 for object;
reserve X,X1,X2,X3,X4,Y for set;

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;
