
theorem for n be non zero Nat holds (Rascal n).2 = n
  proof
    let n be non zero Nat;
    reconsider f = Rascal n as (n+1)-element FinSequence;
    A1: dom f = Seg (n + 1) by Def1;
    1 + 1 <= n + 1 by XREAL_1:6,NAT_1:14; then
    1 + 1 in dom f by A1; then
    f.2 = (2-1)*(n + 1 - 2) + 1 by Def1;
    hence thesis;
  end;
