thus ( len pD = width M implies ex M1 being Matrix of n,m,D st
( len M1 = len M & width M1 = width M & ( for i, j being Nat st [i,j] in Indices M holds
( ( i <> l implies M1 * (i,j) = M * (i,j) ) & ( i = l implies M1 * (l,j) = pD . j ) ) ) ) ) :: thesis: ( not len pD = width M implies ex b₁ being Matrix of n,m,D st b₁ = M )

proof

reconsider M9 = M as Matrix of len M, width M,D by MATRIX_0:51;
reconsider n1 = n, m1 = m as Element of NAT by ORDINAL1:def 12;
defpred S₁[ set , set , set ] means for i, j being Nat st i = $1 & j = $2 holds
( ( i <> l implies $3 = M * (i,j) ) & ( i = l implies $3 = pD . j ) );
assume A1: len pD = width M ; :: thesis: ex M1 being Matrix of n,m,D st
( len M1 = len M & width M1 = width M & ( for i, j being Nat st [i,j] in Indices M holds
( ( i <> l implies M1 * (i,j) = M * (i,j) ) & ( i = l implies M1 * (l,j) = pD . j ) ) ) )
A2: for i, j being Nat st [i,j] in [:(Seg n1),(Seg m1):] holds
ex x being Element of D st S₁[i,j,x]

proof

let i, j be Nat; :: thesis: ( [i,j] in [:(Seg n1),(Seg m1):] implies ex x being Element of D st S₁[i,j,x] )
assume A3: [i,j] in [:(Seg n1),(Seg m1):] ; :: thesis: ex x being Element of D st S₁[i,j,x]

now :: thesis: ( ( i = l & ex x being Element of D st S₁[i,j,x] ) or ( i <> l & ex x being Element of D st S₁[i,j,x] ) )

per cases ( i = l or i <> l ) ;

case A4: i = l ; :: thesis: ex x being Element of D st S₁[i,j,x]

A5: rng pD c= D by FINSEQ_1:def 4;
n1 <> 0 by A3, ZFMISC_1:87;
then len pD = m by A1, MATRIX_0:23;
then j in Seg (len pD) by A3, ZFMISC_1:87;
then j in dom pD by FINSEQ_1:def 3;
then A6: pD . j in rng pD by FUNCT_1:def 3;
S₁[i,j,pD . j] by A4;
hence ex x being Element of D st S₁[i,j,x] by A6, A5; :: thesis: verum

end;

case i <> l ; :: thesis: ex x being Element of D st S₁[i,j,x]

then S₁[i,j,M * (i,j)] ;
hence ex x being Element of D st S₁[i,j,x] ; :: thesis: verum

end;

end;

end;

hence ex x being Element of D st S₁[i,j,x] ; :: thesis: verum

end;

consider M1 being Matrix of n1,m1,D such that
A7: for i, j being Nat st [i,j] in Indices M1 holds
S₁[i,j,M1 * (i,j)] from MATRIX_0:sch 2(A2);
reconsider M1 = M1 as Matrix of n,m,D ;
take M1 ; :: thesis: ( len M1 = len M & width M1 = width M & ( for i, j being Nat st [i,j] in Indices M holds
( ( i <> l implies M1 * (i,j) = M * (i,j) ) & ( i = l implies M1 * (l,j) = pD . j ) ) ) )

A8: now :: thesis: ( len M = len M1 & width M1 = width M )

per cases ( n = 0 or n > 0 ) ;

suppose A9: n = 0 ; :: thesis: ( len M = len M1 & width M1 = width M )

then len M1 = 0 by MATRIX_0:def 2;
then A10: width M1 = 0 by MATRIX_0:def 3;
len M = 0 by A9, MATRIX_0:def 2;
hence ( len M = len M1 & width M1 = width M ) by A9, A10, MATRIX_0:def 2, MATRIX_0:def 3; :: thesis: verum

end;

suppose A11: n > 0 ; :: thesis: ( len M = len M1 & width M = width M1 )

then A12: width M = m by MATRIX_0:23;
len M = n by A11, MATRIX_0:23;
hence ( len M = len M1 & width M = width M1 ) by A11, A12, MATRIX_0:23; :: thesis: verum

end;

end;

end;

Indices M9 = Indices M1 by MATRIX_0:26;
hence ( len M1 = len M & width M1 = width M & ( for i, j being Nat st [i,j] in Indices M holds
( ( i <> l implies M1 * (i,j) = M * (i,j) ) & ( i = l implies M1 * (l,j) = pD . j ) ) ) ) by A7, A8; :: thesis: verum

end;

thus ( not len pD = width M implies ex b₁ being Matrix of n,m,D st b₁ = M ) ; :: thesis: verum