reserve a,b,i,j,k,l,m,n for Nat;

theorem
  for D be non empty set, r be D-valued FinSequence st len r = i+j
  ex p,q be D-valued FinSequence st len p = i & len q = j & r = p^q
  proof
    let D be non empty set, r be D-valued FinSequence such that
    A1: len r = i+j;
    reconsider r1 = r as FinSequence of D by NEWTON02:103;
    ex p1,q1 be FinSequence of D st
    len p1 = i & len q1 = j & r1 = p1^q1 by A1,FINSEQ_2:23;
    hence thesis;
  end;
