docs: def send_getheaders( self, prev_block_hash ):
          """
          Request block headers from a particular block hash.
          Will receive up to 2000 blocks, starting with the block *after*
          the given block hash (prev_block_hash)
          """
  
          getheaders = GetHeaders()
  
          getheaders.add_block_hash( prev_block_hash )
  
          log.debug("send getheaders")
          self.send_message( getheaders )