docs: def prefix(self, prefix, lowercase=True):
          ''' Returns a dictionary of keys with the same prefix.
              Compat with kr/env, lowercased.
  
              > xdg = env.prefix('XDG_')
  
              > for key, value in xdg.items():
                   print('%-20s' % key, value[:6], '…')
              config_dirs      /etc/x…
              current_desktop  MATE
              data_dirs        /usr/s…
              …
          '''
          env_subset = {}
          for key in self._envars.keys():
              if key.startswith(prefix):
                  new_key = key[len(prefix):]  # cut front
                  new_key = new_key.lower() if lowercase else new_key
                  env_subset[new_key] = str(self._envars[key]) # str strips Entry
  
          return Environment(environ=env_subset,
                             sensitive=self._sensitive,
                             blankify=self._blankify,
                             noneify=self._noneify,
                             writable=self._writable,
                            )