let b be set ; :: thesis: for X1, X2, X3, X4 being non empty set
for x being Element of [:X1,X2,X3,X4:] holds
( b = x `2_4 iff for x1 being Element of X1
for x2 being Element of X2
for x3 being Element of X3
for x4 being Element of X4 st x = [x1,x2,x3,x4] holds
b = x2 )
let X1, X2, X3, X4 be non empty set ; :: thesis: for x being Element of [:X1,X2,X3,X4:] holds
( b = x `2_4 iff for x1 being Element of X1
for x2 being Element of X2
for x3 being Element of X3
for x4 being Element of X4 st x = [x1,x2,x3,x4] holds
b = x2 )
let x be Element of [:X1,X2,X3,X4:]; :: thesis: ( b = x `2_4 iff for x1 being Element of X1
for x2 being Element of X2
for x3 being Element of X3
for x4 being Element of X4 st x = [x1,x2,x3,x4] holds
b = x2 )
thus ( b = x `2_4 implies for x1 being Element of X1
for x2 being Element of X2
for x3 being Element of X3
for x4 being Element of X4 st x = [x1,x2,x3,x4] holds
b = x2 ) :: thesis: ( ( for x1 being Element of X1
for x2 being Element of X2
for x3 being Element of X3
for x4 being Element of X4 st x = [x1,x2,x3,x4] holds
b = x2 ) implies b = x `2_4 )

proof

A1: x = [(x `1_4),(x `2_4),(x `3_4),(x `4_4)] ;
assume A2: b = x `2_4 ; :: thesis: for x1 being Element of X1
for x2 being Element of X2
for x3 being Element of X3
for x4 being Element of X4 st x = [x1,x2,x3,x4] holds
b = x2
let x1 be Element of X1; :: thesis: for x2 being Element of X2
for x3 being Element of X3
for x4 being Element of X4 st x = [x1,x2,x3,x4] holds
b = x2
let x2 be Element of X2; :: thesis: for x3 being Element of X3
for x4 being Element of X4 st x = [x1,x2,x3,x4] holds
b = x2
let x3 be Element of X3; :: thesis: for x4 being Element of X4 st x = [x1,x2,x3,x4] holds
b = x2
let x4 be Element of X4; :: thesis: ( x = [x1,x2,x3,x4] implies b = x2 )
assume x = [x1,x2,x3,x4] ; :: thesis: b = x2
hence b = x2 by A2, A1, XTUPLE_0:5; :: thesis: verum

end;

thus ( ( for x1 being Element of X1
for x2 being Element of X2
for x3 being Element of X3
for x4 being Element of X4 st x = [x1,x2,x3,x4] holds
b = x2 ) implies b = x `2_4 ) by MCART_1:76; :: thesis: verum