  A graceful difference labeling (gdl for short) of a directed graph G with
vertex set V is a bijection f between V and {1,...,|V|} such that, when each
arc uv is assigned the difference label f(v)-f(u), the resulting arc labels are
distinct. We conjecture that all disjoint unions of circuits have a gdl, except
in two particular cases. We prove partial results which support this
conjecture.
