code: func (hc *halfConn) freeBlock(b *block) {
  	b.link = hc.bfree
  	hc.bfree = b
  }
docs: // freeBlock returns a block to hc's free list.
  // The protocol is such that each side only has a block or two on
  // its free list at a time, so there's no need to worry about
  // trimming the list, etc.