theorem Th2:
  for i,j,n being Nat, M being Matrix of n,K st i in dom M holds
  len Deleting(M,i,j) = n-'1
proof
  let i,j,n be Nat, M be Matrix of n,K;
  assume
A1: i in dom M;
A2: len M = n by MATRIX_0:def 2;
  thus len Deleting(M,i,j)= len DelLine(M,i) by MATRIX_0:def 13
    .= n-'1 by A1,A2,Th1;
end;
