theorem
 for X1,X2,X3,X4 being non empty set
 for x being Element of [:X1,X2,X3,X4:]
 for x1,x2,x3,x4 being set st x = [x1,
  x2,x3,x4] holds x`1_4 = x1 & x`2_4 = x2 & x`3_4 = x3 & x`4_4 = x4;
