let K be Field; :: thesis: for M1, M2 being Matrix of K st len M1 = len M2 & width M1 = width M2 holds
M1 - (M1 - M2) = M2
let M1, M2 be Matrix of K; :: thesis: ( len M1 = len M2 & width M1 = width M2 implies M1 - (M1 - M2) = M2 )
assume A1: ( len M1 = len M2 & width M1 = width M2 ) ; :: thesis: M1 - (M1 - M2) = M2
A2: ( len (- M1) = len M1 & width (- M1) = width M1 ) by MATRIX_3:def 2;
A3: ( len (- M2) = len M2 & width (- M2) = width M2 ) by MATRIX_3:def 2;

per cases ( len M1 > 0 or len M1 = 0 ) by NAT_1:3;

suppose A4: len M1 > 0 ; :: thesis: M1 - (M1 - M2) = M2

then A5: M2 is Matrix of len M1, width M1,K by A1, MATRIX_1:20;
A6: M1 is Matrix of len M1, width M1,K by A4, MATRIX_1:20;
A7: len (0. K,(len M1),(width M1)) = len M1 by MATRIX_1:def 3;
then A8: width (0. K,(len M1),(width M1)) = width M1 by A4, MATRIX_1:20;
M1 - (M1 - M2) = M1 + ((- M1) + (- (- M2))) by A1, A3, Th12
.= M1 + ((- M1) + M2) by Th1
.= (M1 + (- M1)) + M2 by A1, A2, MATRIX_3:5
.= (0. K,(len M1),(width M1)) + M2 by A6, MATRIX_3:7
.= M2 + (0. K,(len M1),(width M1)) by A1, A7, A8, MATRIX_3:4
.= M2 by A5, MATRIX_3:6 ;
hence M1 - (M1 - M2) = M2 ; :: thesis: verum

end;

suppose A9: len M1 = 0 ; :: thesis: M1 - (M1 - M2) = M2

then len (M1 - (M1 - M2)) = 0 by MATRIX_3:def 3;
hence M1 - (M1 - M2) = M2 by A1, A9, CARD_2:83; :: thesis: verum

end;

end;