Access.pop
You're seeing just the callback
pop, go back to Access module for more information.
Specs
pop(data, key()) :: {value(), data} when data: container() | any_container()
Invoked to "pop" the value under key out of the given data structure.
When key exists in the given structure data, the implementation should
return a {value, new_data} tuple where value is the value that was under
key and new_data is term without key.
When key is not present in the given structure, a tuple {value, data}
should be returned, where value is implementation-defined.
See the implementations for Map.pop/3 or Keyword.pop/3 for more examples.