noisy. These instructions involve unicode to dictionary lookups
      which Python uses to map variable names. FunctionCounts is
      generally a content agnostic container, however this is
      sufficiently important for obtaining reliable results to warrant
      an exception.
  Return type:
     *FunctionCounts*

filter(filter_fn)
  Keep only the elements where *filter_fn* applied to function
  name returns True.

  Return type:
     *FunctionCounts*

transform(map_fn)
  Apply *map_fn* to all of the function names.

  This can be used to regularize function names (e.g. stripping
  irrelevant parts of the file path), coalesce entries by mapping
  multiple functions to the same name (in which case the counts
  are added together), etc.

  Return type:
     *FunctionCounts*
