code: func (c *Compiler) RuleIndex(path Ref) RuleIndex {
  	r, ok := c.ruleIndices.Get(path)
  	if !ok {
  		return nil
  	}
  	return r.(RuleIndex)
  }
docs: // RuleIndex returns a RuleIndex built for the rule set referred to by path.
  // The path must refer to the rule set exactly, i.e., given a rule set at path
  // data.a.b.c.p, refs data.a.b.c.p.x and data.a.b.c would not return a
  // RuleIndex built for the rule.