reserve i,n for Nat,
  K for Field,
  M1,M2,M3,M4 for Matrix of n,K;

theorem
  M1 is_congruent_Matrix_of M2 implies M1+M1 is_congruent_Matrix_of M2+M2
proof
  assume M1 is_congruent_Matrix_of M2; then
  consider M4 be Matrix of n,K such that
A3: M4 is invertible and
A4: M1=M4@*M2*M4;
A5: width (M4@*M2)=n by MATRIX_0:24;
  take M4;
A6: len M4=n & len (M4@*M2)=n by MATRIX_0:24;
A7: len M2=n & width M2=n by MATRIX_0:24;
  len (M4@)=n & width (M4@)=n by MATRIX_0:24;
  then M4@*(M2+M2)*M4=(M4@*M2+M4@*M2)*M4 by A7,MATRIX_4:62
    .=M1+M1 by A4,A6,A5,MATRIX_4:63;
  hence thesis by A3;
end;
