_getHash(nodeArrays) {
    var str = nodeArrays
      .reduce((a, b) => a.concat(b))
      .map(node => node.name || node.type)
      .join(':');

    return crypto.createHash('sha1').update(str).digest('hex');
  }
Generates a hash for a match.

@private

@param   {Node[][]} nodeArrays   Multi-dimensional array of nodes
@returns {String}