= Graceful Graphs =

== Problem Description ==

A graph (V,E) is called a graceful graph if its
vertices V can be labelled with distinct unsigned integers, its edges E
can be labelled with the absolute differences between vertex labels, and
the edge labels are in the range {1..|E|}. The task is to determine the
existence of a graceful labelling of a graph.

The edges of the graph are provided by instances of the predicate edge/2.
If a graceful labelling exists, a witness containing vertex labels encoded
in the predicate value/2 has to be provided. value(X,I) means the vertex X
is labelled with the unsigned integer I.

== Predicates ==

 * '''Input''': {{{edge/2}}}

 * '''Output''': {{{value/2}}}

== Input format ==


== Output format ==


== Example(s) ==


== Problem Peculiarities ==

'''Type''': Search
'''Competition''': Both
'''Complexity''': NP

== Notes and Updates ==



== Author(s) ==
 * Author: Christian Drescher
  * Affiliation: NICTA, University of New South Wales, Australia