Rollup.js : How to choose a single export from a js file with multiple exports?

I have a single .js file containing multiple exports. It looks like this:

// some-functions.js

export function plusOne(value){
    return value + 1
}

export function minusOne(value){
    return value - 1
}

Then in my rollup.config.js I want to import one of these functions (plusOne) and create an iife from it.

// rollup.config.js

export default {
    input:'./some-functions.js',
    /* Named export goes here? */
    output:{
        name:'plusone',
        file:'./rolled-up-functions/plus-one.js',
        format: 'iife'
    }
}

Is there a way of doing this in rollup? If so, how?

And if not, can anyone recommend a way of doing this using some other tool?