public Iterator<? extends Property<?>> legalVertexProperties(final Vertex vertex) {
        return null == this.vertexPropertyFilter ?
            vertex.properties() :
            TraversalUtil.applyAll(vertex, this.vertexPropertyFilter);
    }
Returns an iterator of legal vertex properties. If no vertex property
filter is provided, then all vertex properties are returned.

@param vertex the vertex whose legal vertex properties are to be accessed.
@return an iterator of vertex properties that are {@link Legal#YES}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/GraphFilter.java