nancy[try].js

class Nancy {
    constructor(executor) {
        const tryCall = callback => Nancy.try(() => callback(this.value));
        const members = {
            [states.resolved]: {
                ...
                then: trycall
            },
            ...
        };
        ...
    }
    ...

    static try(callback) {
        return new Nancy(resolve => resolve(callback()));
    }
}