theorem
  for M1,M2 being Matrix of D st width M1>0 & width M2>0 holds M1=M2 iff
  M1@=M2@ & width M1 = width M2
proof
  let M1,M2 be Matrix of D;
  assume that
A1: width M1>0 and
A2: width M2>0;
  thus M1=M2 implies M1@=M2@ & width (M1) =width (M2);
  assume that
A3: M1@=M2@ and
A4: width (M1) =width (M2);
  len M1=width (M1@) by A1,Th54;
  then
A5: len M1=len M2 by A2,A3,Th54;
A6: Indices M2=[:dom M2,Seg width M2:];
  for i,j st [i,j] in Indices M1 holds M1*(i,j) = M2*(i,j)
  proof
    let i,j;
    assume
A7: [i,j] in Indices M1;
    dom M1 = Seg len M2 by A5,FINSEQ_1:def 3
      .= dom M2 by FINSEQ_1:def 3;
    then M2@*(j,i)=M2*(i,j) by A4,A6,A7,Def6;
    hence thesis by A3,A7,Def6;
  end;
  hence thesis by A4,A5,Th21;
end;
