Update Visitor-and-Patch.md

Author: antonmedv

docs/Visitor-and-Patch.md

@@ -48,6 +48,60 @@ program, err := expr.Compile(code, expr.Patch(&visitor{}))
 ```
 
 This can be useful for some edge cases, there you want to extend functionality of **Expr** language. 
+In next example we are going to replace expression `list[-1]` with `list[len(list)-1]`.
+
+```go
+package main
+
+import (
+	"fmt"
+
+	"github.com/antonmedv/expr"
+	"github.com/antonmedv/expr/ast"
+)
+
+func main() {
+	env := map[string]interface{}{
+		"list": []int{1, 2, 3},
+	}
+
+	code := `list[-1]` // will output 3
+
+	program, err := expr.Compile(code, expr.Env(env), expr.Patch(&patcher{}))
+	if err != nil {
+		panic(err)
+	}
+
+	output, err := expr.Run(program, env)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Print(output)
+}
+
+type patcher struct{}
+
+func (p *patcher) Enter(_ *ast.Node) {}
+func (p *patcher) Exit(node *ast.Node) {
+	n, ok := (*node).(*ast.IndexNode)
+	if !ok {
+		return
+	}
+	unary, ok := n.Index.(*ast.UnaryNode)
+	if !ok {
+		return
+	}
+	if unary.Operator == "-" {
+		ast.Patch(&n.Index, &ast.BinaryNode{
+			Operator: "-",
+			Left:     &ast.BuiltinNode{Name: "len", Arguments: []ast.Node{n.Node}},
+			Right:    unary.Node,
+		})
+	}
+
+}
+```
+
 Type information is also available. Here is an example, there all `fmt.Stringer` interface automatically 
 converted to `string` type.