
theorem Th21:
  for F being finite set, A being FinSequence of bool F,
      i being Element of NAT,
      x being set st i in dom A holds Cut (A,i,x) is Reduction of A
proof
  let F be finite set, A be FinSequence of bool F,
      i be Element of NAT, x be set;
  assume i in dom A;
  then Cut (A,i,x) is Reduction of A,i by Th20;
  hence thesis by Th19;
end;
