code: func (rule *Rule) Path() Ref {
  	if rule.Module == nil {
  		panic("assertion failed")
  	}
  	return rule.Module.Package.Path.Append(StringTerm(string(rule.Head.Name)))
  }
docs: // Path returns a ref referring to the document produced by this rule. If rule
  // is not contained in a module, this function panics.