function done( error ) {
		if ( error ) {
			return cb( error );
		}
		debug( 'Finished analyzing file %d of %d.', idx+1, nfiles );
		if ( idx === nfiles-1 ) {
			if ( opts.cumulative ) {
				debug( 'Finished analysis. Returning cumulative results...' );
				return cb( null, acc() );
			}
			debug( 'Finished analysis. Returning results...' );
			return cb( null, results );
		}
		next();
	}
Callback invoked upon analyzing a file.

@private
@param {(Error|null)} [error] - error object
@returns {void}