public static VertexProgramRestrictionStrategy instance() {
        return INSTANCE;
    }
Detects the presence of a {@link VertexProgramStrategy} and throws an {@link IllegalStateException} if it is found.

@author Marc de Lignie
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/traversal/strategy/verification/VertexProgramRestrictionStrategy.java