code: function NixAttrReference(args) {
      this.attrSetExpr = args.attrSetExpr;
      this.refExpr = args.refExpr;
      this.orExpr = args.orExpr;
  }
docs: Creates a new NixAttrReference instance.
  
  @class NixAttrReference
  @extends NixObject
  @classdesc Captures the abstract syntax of a Nix of an expression yielding an
  attribute set and an expression yielding an attribute name that references a
  member of the former attribute set.
  
  @constructor
  @param {Object} args Arguments to this function
  @param {Mixed} args.attrSetExpr An object representing an expression that yields an attribute set
  @param {Mixed} args.refExpr An object representing an expression that yields an attribute name
  @param {Mixed} args.orExpr An optional object representing an expression that gets evaluated if the reference does not exist.