func (st *state) findArgumentPack(a AST) *ArgumentPack {
	var seen []AST
	var ret *ArgumentPack
	a.Traverse(func(a AST) bool {
		if ret != nil {
			return false
		}
		switch a := a.(type) {
		case *TemplateParam:
			if a.Template == nil || a.Index >= len(a.Template.Args) {
				return true
			}
			if pack, ok := a.Template.Args[a.Index].(*ArgumentPack); ok {
				ret = pack
				return false
			}
		case *PackExpansion, *Closure, *Name:
			return false
		case *TaggedName, *Operator, *BuiltinType, *FunctionParam:
			return false
		case *UnnamedType, *FixedType, *DefaultArg:
			return false
		}
		for _, v := range seen {
			if v == a {
				return false
			}
		}
		seen = append(seen, a)
		return true
	})
	return ret
}
func (st *state) findArgumentPack(a AST) *ArgumentPack {
	var seen []AST
	var ret *ArgumentPack
	a.Traverse(func(a AST) bool {
		if ret != nil {
			return false
		}
		switch a := a.(type) {
		case *TemplateParam:
			if a.Template == nil || a.Index >= len(a.Template.Args) {
				return true
			}
			if pack, ok := a.Template.Args[a.Index].(*ArgumentPack); ok {
				ret = pack
				return false
			}
		case *PackExpansion, *Closure, *Name:
			return false
		case *TaggedName, *Operator, *BuiltinType, *FunctionParam:
			return false
		case *UnnamedType, *FixedType, *DefaultArg:
			return false
		}
		for _, v := range seen {
			if v == a {
				return false
			}
		}
		seen = append(seen, a)
		return true
	})
	return ret
}
// findArgumentPack walks the AST looking for the argument pack for a
// pack expansion.  We find it via a template parameter.