code: func (mod *Module) RuleSet(name Var) RuleSet {
  	rs := NewRuleSet()
  	for _, rule := range mod.Rules {
  		if rule.Head.Name.Equal(name) {
  			rs.Add(rule)
  		}
  	}
  	return rs
  }
docs: // RuleSet returns a RuleSet containing named rules in the mod.