BIND operator broken since 7.0

Describe the bug Bind operator working wrong since 7.0

To Reproduce Just use a query with BIND inside one of the examples (here I chose the one about using a storage based on N3.js).

'use strict'

const { Parser, Store } = require('n3')
const { HashMapDataset, Graph, PlanBuilder } = require('sparql-engine')

// Format a triple pattern according to N3 API:
// SPARQL variables must be replaced by `null` values
function formatTriplePattern (triple) {
  let subject = null
  let predicate = null
  let object = null
  if (!triple.subject.startsWith('?')) {
    subject = triple.subject
  }
  if (!triple.predicate.startsWith('?')) {
    predicate = triple.predicate
  }
  if (!triple.object.startsWith('?')) {
    object = triple.object
  }
  return { subject, predicate, object }
}

class N3Graph extends Graph {
  constructor () {
    super()
    this._store = Store()
  }

  insert (triple) {
    return new Promise((resolve, reject) => {
      try {
        this._store.addTriple(triple.subject, triple.predicate, triple.object)
        resolve()
      } catch (e) {
        reject(e)
      }
    })
  }

  delete (triple) {
    return new Promise((resolve, reject) => {
      try {
        this._store.removeTriple(triple.subject, triple.predicate, triple.object)
        resolve()
      } catch (e) {
        reject(e)
      }
    })
  }

  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: <http://xmlns.com/foaf/0.1/> .
  @prefix : <http://example.org#> .
  :a foaf:name "a" .
  :b foaf:name "b" .
`).forEach(t => {
  graph._store.addTriple(t)
})

const query = `
  PREFIX foaf: <http://xmlns.com/foaf/0.1/>
  SELECT ?s ?name
  WHERE {
    BIND(<http://example.org#a> as ?s)
    ?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 => {
  console.log('Found solution:', bindings.toObject())
}, err => {
  console.error('error', err)
}, () => {
  console.log('Query evaluation complete!')
})

Expected behavior On 6.0 and below, I get the expected result:

Found solution: { '?s': 'http://example.org#a', '?name': '"a"' }
Query evaluation complete!

While on 7.0 and above, I get:

Found solution: { '?s': 'http://example.org#a', '?name': '"a"' }
Found solution: { '?s': 'http://example.org#a', '?name': '"b"' }
Query evaluation complete!

Callidon / sparql-engine

BIND operator broken since 7.0 #78