x/tools/go/ast/inspector: add (*Cursor).Valid() bool

// This program demonstrates how the new `Valid()` method on `Cursor` would be used.
// It defines a minimal `Cursor` type with a `Valid()` method to illustrate the API.
// In real code, you would import "golang.org/x/tools/go/ast/inspector" instead.
package main

import (
	"fmt"
)

// Cursor is a stand‑in for the real inspector.Cursor type.
type Cursor struct {
	inspector *int // nil means the cursor is invalid
}

// Valid reports whether the cursor is valid.
func (c Cursor) Valid() bool {
	return c.inspector != nil
}

func main() {
	// An invalid cursor: inspector field is nil.
	var bad Cursor
	fmt.Println("bad cursor valid?", bad.Valid()) // false

	// A valid cursor: inspector field points to something.
	x := 1
	good := Cursor{inspector: &x}
	fmt.Println("good cursor valid?", good.Valid()) // true
}