reserve i,j for Nat;

theorem Th55:
  for K being set, A,B being Matrix of K st len A=len B & width A=
  width B holds Indices A=Indices B
proof
  let K be set,A,B be Matrix of K;
A1: dom A = Seg len A by FINSEQ_1:def 3;
  assume len A=len B & width A=width B;
  hence thesis by A1,FINSEQ_1:def 3;
end;
