theorem
  for M1,M2 being Matrix of D st width M1>0 & width M2>0 & M1@=M2@ holds M1=M2
proof
  let M1,M2 be Matrix of D;
  assume width M1>0 & width M2>0;
  then
A1: width (M1@)=len M1 & width (M2@)=len M2 by Th54;
  assume M1@=M2@;
  hence thesis by A1,Th53;
end;
