public default Iterator<Vertex> head(final String location, final Class readerClass) {
        return this.head(location, readerClass, Integer.MAX_VALUE);
    }
Get the vertices at the specified graph location.

@param location    the location of the graph (or the root location and search will be made)
@param readerClass the class of the parser that understands the graph format
@return an iterator of vertices.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/io/Storage.java