func (st *state) setTemplate(a AST, tmpl *Template) {
	var seen []AST
	a.Traverse(func(a AST) bool {
		switch a := a.(type) {
		case *TemplateParam:
			if a.Template != nil {
				if tmpl != nil {
					st.fail("duplicate template parameters")
				}
				return false
			}
			if tmpl == nil {
				st.fail("cast template parameter not in scope of template")
			}
			if a.Index >= len(tmpl.Args) {
				st.fail(fmt.Sprintf("cast template index out of range (%d >= %d)", a.Index, len(tmpl.Args)))
			}
			a.Template = tmpl
			return false
		default:
			for _, v := range seen {
				if v == a {
					return false
				}
			}
			seen = append(seen, a)
			return true
		}
	})
}
func (st *state) setTemplate(a AST, tmpl *Template) {
	var seen []AST
	a.Traverse(func(a AST) bool {
		switch a := a.(type) {
		case *TemplateParam:
			if a.Template != nil {
				if tmpl != nil {
					st.fail("duplicate template parameters")
				}
				return false
			}
			if tmpl == nil {
				st.fail("cast template parameter not in scope of template")
			}
			if a.Index >= len(tmpl.Args) {
				st.fail(fmt.Sprintf("cast template index out of range (%d >= %d)", a.Index, len(tmpl.Args)))
			}
			a.Template = tmpl
			return false
		default:
			for _, v := range seen {
				if v == a {
					return false
				}
			}
			seen = append(seen, a)
			return true
		}
	})
}
// setTemplate sets the Template field of any TemplateParam's in a.
// This handles the forward referencing template parameters found in
// cast operators.