reserve a,b for Real,
  i,j,n for Nat,
  M,M1,M2,M3,M4 for Matrix of n, REAL;

theorem Th5:
  Indices M = Indices |:M:|
proof
A1: Seg len M = dom M by FINSEQ_1:def 3;
  len |:M:|= len M & width |:M:| =width M by Def7;
  hence thesis by A1,FINSEQ_1:def 3;
end;
