reserve i,j,k,n for Nat;
reserve D for non empty set,
  p for Element of D,
  f,g for FinSequence of D;
reserve D for set,
  f for FinSequence of D;

theorem
  len f >= 2 implies f|2 = <*f/.1,f/.2*>
proof
  set d1 = f/.1, d2 = f/.2;
  assume
A1: len f >= 2;
  then
A2: len(f|2) = 2 by FINSEQ_1:59;
  reconsider D as non empty set by A1;
  reconsider f as FinSequence of D;
  2 in dom(f|2) by A2,FINSEQ_3:25;
  then
A3: d2 = (f|2)/.2 by FINSEQ_4:70
    .= (f|2).2 by A2,FINSEQ_4:15;
  1 in dom(f|2) by A2,FINSEQ_3:25;
  then d1 = (f|2)/.1 by FINSEQ_4:70
    .= (f|2).1 by A2,FINSEQ_4:15;
  hence thesis by A2,A3,FINSEQ_1:44;
end;
