A mutable ItemStack. withTag withAmount damageItem etc. Changes (such as count or tag changes) will modify and return the ItemStack itself instead of a new ItemStack with the changed properties.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.item.IMutableItemStack;

You can get a IMutableItemStack by using IItemStack's mutable() method. like so:

ZenScript
 Copy
<minecraft:dirt>.mutable()

IMutableItemStack extends IItemStack and is able to call all of its methods/getters/setters as well.

Besides withAmount, you can easily call these methods below to change the count of item.

ZenMethod	Parameter Type	Description
ZenMethod grow(count)	Parameter Type int	Description Increases the stack's count by the count provided
ZenMethod shrink(count)	Parameter Type int	Description Decreases the stack's count by the count provided

The method attempts to damage the stack with an optional player. Returns if the stack is damaged successfully.

boolean attemptDamageItem(int amount, @Optional IPlayer player);

IItemStack copy();

The copy method will returns a new immutable ItemStack with the same properties. If you are certain that the stack shouldn't be changed anymore and want to avoid unexpected item changes, you will need to use the method.