code: func (t *Pot) getPos(po int) (n *Pot, i int) {
  	for i, n = range t.bins {
  		if po > n.po {
  			continue
  		}
  		if po < n.po {
  			return nil, i
  		}
  		return n, i
  	}
  	return nil, len(t.bins)
  }
docs: // getPos called on (n) returns the forking node at PO n and its index if it exists
  // otherwise nil
  // caller is supposed to hold the lock