#### Answer

Inverse action: empty the cup.

#### Work Step by Step

In loose terms,
let f( empty cup )= full cup
We want the inverse to be defined on full cups
g( full cup )= empty cup
So,
g(f ( empty cup )= empty cup
and
f(g( full cup )= full cup
[ f(g(x))=x and g(f(x))=x ]
Inverse action: empty the cup