Tractatus Logico-Philosophicus by Ludwig Wittgenstein + comments  by Maarten Maartensz   

5.31 The schemata in 4.31 have a meaning even when 'p', 'q', 'r', etc. are not elementary propositions. And it is easy to see that the propositional sign in 4.442 expresses a single truth-function of elementary propositions even when 'p' and 'q' are truth-functions of elementary propositions.

As explained under 4.442  W. was wrong about "|-", which in fact is (like) a modality.