
theorem
  for p being non empty Graph-yielding FinSequence
  st p.1 is Tree-like &
    for n being Element of dom p st n <= len p - 1
    ex v1,e,v2 being object st p.(n+1) is addAdjVertex of p.n,v1,e,v2
  holds p.len p is Tree-like
proof
  let p be non empty Graph-yielding FinSequence;
  assume that A1: p.1 is Tree-like and
    A2: for n being Element of dom p st n <= len p - 1 holds
      ex v1,e,v2 being object st p.(n+1) is addAdjVertex of p.n,v1,e,v2;
  defpred Q[Nat] means $1 <= len p implies ex k being Element of dom p
    st $1 = k & p.k is Tree-like;
  A3: Q[1]
  proof
    assume 1 <= len p;
    then reconsider k = 1 as Element of dom p by FINSEQ_3:25;
    take k;
    thus thesis by A1;
  end;
  A4: for m being non zero Nat st Q[m] holds Q[m+1]
  proof
    let m be non zero Nat;
    assume A5: Q[m];
    assume A6: m+1 <= len p;
    0+1 <= m+1 by XREAL_1:6;
    then reconsider k = m+1 as Element of dom p by A6, FINSEQ_3:25;
    take k;
    thus m+1 = k;
    m+1-1 <= len p - 0 by A6, XREAL_1:13;
    then consider k0 being Element of dom p such that
      A7: m = k0 & p.k0 is Tree-like by A5;
    m+1-1 <= len p - 1 by A6, XREAL_1:9;
    then consider v1,e,v2 being object such that
      A8: p.(k0+1) is addAdjVertex of p.k0,v1,e,v2 by A2, A7;
    thus thesis by A7, A8;
  end;
  A9: for m being non zero Nat holds Q[m] from NAT_1:sch 10(A3,A4);
  consider k being Element of dom p such that
    A10: len p = k & p.k is Tree-like by A9;
  thus thesis by A10;
end;
