reserve x,y,X for set,
  i,j,k,m,n for Nat,
  p for FinSequence of X,
  ii for Integer;

theorem Th2:
  1 <= ii & ii <= len p implies p.ii in X
proof
  assume that
A1: 1 <= ii and
A2: ii <= len p;
  reconsider ii as Element of NAT by A1,INT_1:3;
  ii in dom p by A1,A2,FINSEQ_3:25;
  hence thesis by PARTFUN1:4;
end;
