  We compute the class of a divisor on M_{g,n} given as the closure of the
locus of smooth pointed curves [C; x_1,..., x_n] for which \sum d_j x_j has an
effective representative, where d_j are integers summing up to g-1, not all
positive. The techniques used are a vector bundle computation, a pushdown
argument reducing the number of marked points, and the method of test curves.
