Robert Wilber

The black pebble game is played by placing pebbles on, and removing them from, the vertices of a directed acyclic graph according to rules that model the deterministic evaluation of a straight-line program. The black-white pebble game models nondeterministic evaluations. In both there is a dag with vertix indegrees bounded by 2 for which an optimal programs for which nondeterminism reduces the space required to evaluate the programs by more than any constant factor.