reserve x,y,z for object,
  i,j,n,m for Nat,
  D for non empty set,
  s,t for FinSequence,
  a,a1,a2,b1,b2,d for Element of D,
  p, p1,p2,q,r for FinSequence of D;
reserve M,M1,M2 for Matrix of D;

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;
