public boolean legalVertex(final Vertex vertex) {
        return null == this.vertexFilter || TraversalUtil.test(vertex, this.vertexFilter);
    }
Returns true if the provided vertex meets the vertex-filter criteria.
If no vertex filter is provided, then the vertex is considered legal.

@param vertex the vertex to test for legality
@return whether the vertex is {@link Legal#YES}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/GraphFilter.java