How to use the sparql-engine.HashMapDataset function in sparql-engine

db.put([triple1, triple2], () => {
  const graph = new LevelRDFGraph(db)
  const dataset = new HashMapDataset('http://example.org#default', graph)

  const query = `
    PREFIX foaf: 
    SELECT ?name
    WHERE {
      ?s foaf:name ?name .
    }`

  // Creates a plan builder for the RDF dataset
  const builder = new PlanBuilder(dataset)

  // Get an iterator to evaluate the query
  const iterator = builder.build(query)

  // Read results
  iterator.subscribe(bindings => {

})
  }

  find (triple) {
    const { subject, predicate, object } = formatTriplePattern(triple)
    return this._store.getTriples(subject, predicate, object)
  }

  estimateCardinality (triple) {
    const { subject, predicate, object } = formatTriplePattern(triple)
    return Promise.resolve(this._store.countTriples(subject, predicate, object))
  }
}

const graph = new N3Graph()
const dataset = new HashMapDataset('http://example.org#default', graph)

// Load some RDF data into the graph
const parser = new Parser()
parser.parse(`
  @prefix foaf:  .
  @prefix :  .
  :a foaf:name "a" .
  :b foaf:name "b" .
`).forEach(t => {
  graph._store.addTriple(t)
})

const query = `
  PREFIX foaf: 
  SELECT ?name
  WHERE {