function done( error ) {
			if ( error ) {
				return clbk( error );
			}
			debug( 'Processed %d of %d AST nodes.', idx+1, total );
			if ( idx === total-1 ) {
				debug( 'Finished processing file.' );
				return clbk();
			}
			next();
		}
Callback invoked upon processing an AST node.

@private
@param {Error} [error] - error object
@returns {void}