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

assume A1: 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 A2: x = [x1,x2,x3] ; :: thesis: b = x2
x = [(x `1 ),(x `2 ),(x `3 )] by MCART_1:48;
hence b = x2 by A1, A2, MCART_1:28; :: 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_1:70; :: thesis: verum