Pre-condition: object must be held. This condition requires that an object of a command must be held by the actor. If it is not, we will attempt a recursive "take" command on the object.

This condition is useful for commands where the object is to be manipulated in some way, or used to manipulate some other object. For example, the key in "unlock door with key" would normally have to be held.

objHeld :   PreCondition