In the above link, pg describes the problem as being in the fact that he was using "a four variable list to destructure on a five value description of the field."
The above bit of code appears to be the "five value description". It's a function creating a data structure specifically for the particular user and post, making some data showable/not showable and modifiable/not modifiable.
The only time that this particular variable is used is in calls to a 'vars-form function. This function is in app.arc <insert snarky comment about badly disorganized code here>:
The succeeding line has a "(and mod v)" line, which might be correctly "(and mod valid)", where valid is the missing variable in the destructure. <insert snarky comment about pg's Arc-community-building skills here>
