theorem Th23:
  n > 0 implies for M being Matrix of n,m,D holds len M = n &
  width M = m & Indices M = [:Seg n, Seg m:]
proof
  assume
A1: n > 0;
  let M be Matrix of n,m,D;
  Seg len M = dom M & len M = n by Def2,FINSEQ_1:def 3;
  hence thesis by A1,Th20;
end;
