code: function flatten( arr ) {
  	var tmp;
  	var out;
  	var i;
  	var j;
  	
  	out = [];
  	for ( i = 0; i < arr.length; i++ ) {
  		tmp = arr[ i ];
  		if ( tmp ) {
  			for ( j = 0; j < tmp.length; j++ ) {
  				out.push( tmp[ j ] );
  			}
  		}
  	}
  	return out;
  }
docs: FUNCTION: flatten( arr )
  Flattens an array of arrays.
  
  @private
  @param {Array[]} arr - array of arrays
  @returns {Array} 1d array