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(*         *   The Coq Proof Assistant / The Coq Development Team       *)
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(** A constructive proof of a version of Weak König's Lemma over a decidable predicate in the formulation of which infinite paths are treated as predicates. The representation of paths as relations avoid the need for classical logic and unique choice. The decidability condition is sufficient to ensure that some required instance of double negation for disjunction of finite paths holds.  The idea of the proof comes from the proof of the weak König's lemma from separation in second-order arithmetic.  Notice that we do not start from a tree but just from an arbitrary predicate. Original Weak Konig's Lemma is the instantiation of the lemma to a tree *)
(** [is_path_from P n l] means that there exists a path of length [n] from [l] on which [P] does not hold *)
Inductive is_path_from (P:list bool -> Prop) : nat -> list bool -> Prop :=
| here l : ~ P l -> is_path_from P 0 l
| next_left l n : ~ P l -> is_path_from P n (true::l) -> is_path_from P (S n) l
| next_right l n : ~ P l -> is_path_from P n (false::l) -> is_path_from P (S n) l.
(** We give the characterization of is_path_from in terms of a more common arithmetical formula *)

Proposition is_path_from_characterization P n l :
is_path_from P n l <-> exists l', length l' = n /\ forall n', n'<=n -> ~ P (rev (firstn n' l') ++ l).