const BeforeOnce = ( options, fn ) => Before( options, function () {
        return this.tags.check( options.tags ).then( ( status ) => {
            if ( status === 'new' ) {
                return fn.call ( this ).then( () => this.tags.complete( options.tags ) );
            }
        } );
    } );

Converted to couroutine:

    const BeforeOnce = ( options, fn ) => Before( options, Promise.coroutine( function* () {
        let status = yield this.tags.check( options.tags );
        if ( status === 'new' ) {
            yield fn.call( this );
            yield this.tags.complect( options.tags );
        }
    } ) );