let x, y, c be set ; :: thesis: for S being non empty ManySortedSign st S = BitSubtracterWithBorrowStr (x,y,c) holds
( x in the carrier of S & y in the carrier of S & c in the carrier of S )
set S1 = 2GatesCircStr (x,y,c,'xor');
let S be non empty ManySortedSign ; :: thesis: ( S = BitSubtracterWithBorrowStr (x,y,c) implies ( x in the carrier of S & y in the carrier of S & c in the carrier of S ) )
A1: ( x in the carrier of (2GatesCircStr (x,y,c,'xor')) & y in the carrier of (2GatesCircStr (x,y,c,'xor')) ) by FACIRC_1:60;
A2: c in the carrier of (2GatesCircStr (x,y,c,'xor')) by FACIRC_1:60;
assume S = BitSubtracterWithBorrowStr (x,y,c) ; :: thesis: ( x in the carrier of S & y in the carrier of S & c in the carrier of S )
hence ( x in the carrier of S & y in the carrier of S & c in the carrier of S ) by A1, A2, FACIRC_1:20; :: thesis: verum