func countUses(x *ast.Ident, scope []ast.Stmt) int {
	count := 0
	ff := func(n interface{}) {
		if n, ok := n.(ast.Node); ok && refersTo(n, x) {
			count++
		}
	}
	for _, n := range scope {
		walk(n, ff)
	}
	return count
}
func countUses(x *ast.Ident, scope []ast.Stmt) int {
	count := 0
	ff := func(n interface{}) {
		if n, ok := n.(ast.Node); ok && refersTo(n, x) {
			count++
		}
	}
	for _, n := range scope {
		walk(n, ff)
	}
	return count
}
// countUses returns the number of uses of the identifier x in scope.