Actions are NLP++ functions that are specialized to know about particular contexts. For example, the POST Action single assumes that a rule has successfully matched.
Historically, actions preceded NLP++, and many do not allow full NLP++ expressions as arguments. The CODE, PRE, CHECK, and POST regions all have associated actions.
Many actions, such as the PRINT actions, are superseded by the C++ -like << operator and are retained for backward compatibility.
Note: Some actions are slated for review.