reserve a,a1,a2,v,v1,v2,x for object;
reserve V,A for set;
reserve m,n for Nat;
reserve S,S1,S2 for FinSequence;

theorem Th3:
  for f,g being FinSequence holds dom f c= dom g or dom g c= dom f
  proof
    let f,g be FinSequence;
    consider n being Nat such that
A1: dom f = Seg n by FINSEQ_1:def 2;
    consider m being Nat such that
A2: dom g = Seg m by FINSEQ_1:def 2;
    m <= n or n <= m;
    hence thesis by A1,A2,FINSEQ_1:5;
  end;
