Consider a robot whose operation is described by the following PDDL operators:

Op(ACTION:Go(x, y), PRECOND:At(Robot, x), EFFECT:¬At(Robot, x) ∧ At(Robot, y))

Op(ACTION:P ick(o), PRECOND:At(Robot, x) ∧ At(o, x), EFFECT:¬At(o, x) ∧ Holding(o))

Op(ACTION:Drop(o), PRECOND:At(Robot, x) ∧ Holding(o), EFFECT:At(o, x) ∧ ¬Holding(o))

a. The operators allow the robot to hold more than one object. Show how to modify them with an EmptyHand predicate for a robot that can hold only one object. 

b. Assuming that these are the only actions in the world, write a successor-state axiom for EmptyHand.