docs: def get_header_index(self, hdrclass, startidx=0):
          '''
          Return the first index of the header class hdrclass
          starting at startidx (default=0), or -1 if the
          header class isn't found in the list of headers.
          '''
          for hdridx in range(startidx, len(self._headers)):
              if isinstance(self._headers[hdridx], hdrclass):
                  return hdridx
          return -1