  We formulate and prove a toy version of the Penrose inequality. The
formulation mimics the original Penrose inequality in which the scenario is the
following: A shell of null dust collapses in Minkowski space and a marginally
trapped surface forms on it. Through a series of arguments relying on
established assumptions, an inequality relating the area of this surface to the
total energy of the shell is formulated. Then a further reformulation turns the
inequality into a statement relating the area and the outer null expansion of a
class of surfaces in Minkowski space itself. The inequality has been proven to
hold true in many special cases, but there is no proof in general. In the toy
version here presented, an analogous inequality in (2+1)-dimensional anti-de
Sitter space turns out to hold true.
