code: function evaluatePackageAsync(filename, attr, format, callback) {
      var pkgs = require(path.resolve(filename)).pkgs;
      
      slasp.sequence([
          function(callback) {
              pkgs[attr](callback);
          },
          
          function(callback, pkg) {
              var expr = nijs.jsToNix(pkg, format);
              callback(null, expr);
          }
      ], callback);
  }
docs: Imports the given package composition CommonJS module and asynchronously
  evaluates the specified package.
  
  @param {String} filename Path to the package composition CommonJS module
  @param {String} attr Name of the package to evaluate
  @param {Boolean} format Indicates whether to nicely format to expression (i.e. generating whitespaces) or not
  @param {function(Object, Object)} callback Callback that gets invoked with either an error set if the operation failed, or a string containing the generated Nix expression