function onFile( error, file ) {
		if ( error ) {
			debug( 'Encountered an error when reading file: %s (%d of %d). Error: %s', files[ idx ], idx+1, nfiles, error.message );
			return done( error );
		}
		debug( 'Successfully read file: %s (%d of %d).', files[ idx ], idx+1, nfiles );
		if ( opts.cumulative ) {
			debug( 'Updating cumulative summary...' );
			acc( file );
		} else {
			debug( 'Calculating LLOC...' );
			results.push( lloc( file ) );
		}
		done();
	}
Callback invoked upon reading a file.

@private
@param {(Error|null)} error - error object
@param {(string|Buffer)} file - file contents
@returns {void}