code: def isheader(self, line):
          
          i = line.find(':')
          if i > -1:
              return line[:i].lower()
          return None
docs: Determine whether a given line is a legal header.
  
          This method should return the header name, suitably canonicalized.
          You may override this method in order to use Message parsing on tagged
          data in RFC 2822-like formats with special header formats.