code: func (r RuleSet) Register(rule Rule, nodes ...ast.Node) {
  	for _, n := range nodes {
  		t := reflect.TypeOf(n)
  		if rules, ok := r[t]; ok {
  			r[t] = append(rules, rule)
  		} else {
  			r[t] = []Rule{rule}
  		}
  	}
  }
docs: // Register adds a trigger for the supplied rule for the the
  // specified ast nodes.