  Let B(\infty) be the crystal corresponding to the nilpotent part of a
quantized Kac-Moody algebra. We suggest a general way to represent B(\infty) as
the set of integer solutions of a system of linear inequalities. As an
application, we treat in a unified manner all Kac-Moody algebras of rank 2
(sharpening the result by Kashiwara), as well as the algebras of types A_n and
A_{n-1}^{(1)}.
