reserve x,y,z for set;
reserve f,f1,f2,f3 for FinSequence,
  p,p1,p2,p3 for set,
  i,k for Nat;

theorem Th21:
  p1..<*p1,p2,p3*> = 1
proof
A1: for i st 1<=i & i<1 holds <*p1,p2,p3*>.i <> <*p1,p2,p3*>.1;
  len<*p1,p2,p3*> = 3 by FINSEQ_1:45;
  then
A2: 1 in dom<*p1,p2,p3*> by FINSEQ_3:25;
  thus thesis by A2,A1,Th2;
end;
