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

proof

A1: x = [(x `1),(x `2),(x `3)] by MCART_1:44;
assume A2: b = x `2 ; :: thesis: for x1 being Element of X1
for x2 being Element of X2
for x3 being Element of X3 st x = [x1,x2,x3] holds
b = x2
let x1 be Element of X1; :: thesis: for x2 being Element of X2
for x3 being Element of X3 st x = [x1,x2,x3] holds
b = x2
let x2 be Element of X2; :: thesis: for x3 being Element of X3 st x = [x1,x2,x3] holds
b = x2
let x3 be Element of X3; :: thesis: ( x = [x1,x2,x3] implies b = x2 )
assume x = [x1,x2,x3] ; :: thesis: b = x2
hence b = x2 by A2, A1, MCART_1:25; :: thesis: verum

end;

thus ( ( for x1 being Element of X1
for x2 being Element of X2
for x3 being Element of X3 st x = [x1,x2,x3] holds
b = x2 ) implies b = x `2 ) by MCART_1:66; :: thesis: verum