  A graph with n vertices is 1-planar if it can be drawn in the plane such that
each edge is crossed at most once, and is optimal if it has the maximum of 4n-8
edges.
  We show that optimal 1-planar graphs can be recognized in linear time. Our
algorithm implements a graph reduction system with two rules, which can be used
to reduce every optimal 1-planar graph to an irreducible extended wheel graph.
The graph reduction system is non-deterministic, constraint, and non-confluent.
