theorem Th30:
  for D being set for i,j for M being Matrix of D st 1 <= i & i <=
  len M & 1 <= j & j <= width M holds [i,j] in Indices M
proof
  let D be set;
  let i,j;
  let M be Matrix of D;
  assume 1 <= i & i <= len M & 1 <= j & j <= width M;
  then i in dom M & j in Seg width M by FINSEQ_1:1,FINSEQ_3:25;
  hence thesis by ZFMISC_1:87;
end;
