theorem
  for x,y being Element of [:X1,X2,X3,X4:]
   st x`1_4=y`1_4 & x`2_4=y`2_4 & x`3_4=y
  `3_4 & x`4_4=y`4_4 holds x=y
proof
  let x,y be Element of [:X1,X2,X3,X4:];
  [x`1_4,x`2_4,x`3_4,x`4_4]=x;
  hence thesis by AB;
end;
