protected void checkKeyValue(final String key, final Object value) {
        if (!this.memoryKeys.containsKey(key))
            throw GraphComputer.Exceptions.providedKeyIsNotAMemoryComputeKey(key);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
tinkergraph-gremlin/src/main/java/org/apache/tinkerpop/gremlin/tinkergraph/process/computer/TinkerMemory.java