code: def filename( self, node ):
          
          if not node.directory:
              # TODO: any cases other than built-ins?
              return None
          if node.filename == '~':
              # TODO: look up C/Cython/whatever source???
              return None
          return os.path.join(node.directory, node.filename)
docs: Extension to squaremap api to provide "what file is this" information