Non-Negative Integer Linear Congruences

We consider the problem of describing all non-negative integer solutions to a linear congruence in many variables. This question may be reduced to solving the congruence $x_1 + 2x_2 + 3x_3 + ... + (n-1)x_{n-1} \equiv 0 \pmod n$ where values of the unknowns, $x_i$, are sought among the non-negative integers. We consider the monoid of solutions of this equation and prove a conjecture of Elashvili concerning the structure of these solutions. This yields a simple algorithm for generating most (conjecturally all) of the high degree indecomposable solutions of the equation.