This function takes a selector or element, and a predicate, and returns
an array of the hrefs of all `a` elements in the given element matching the
predicate. Useful for manual scraping.

```js
window.q = (x,y=document) => y.querySelector(x)
window.qq = (x,y=document) => Array.from(y.querySelectorAll(x))

function get_matching(elt,pred = x => true) {
    if( typeof elt == "string" ) elt = q(elt)
    return qq("a",elt).filter(x => pred(x.href)).map(x => x.href)
}
```

You could of course make this more general, and make the above a
special case:
```js
window.q = (x,y=document) => y.querySelector(x)
window.qq = (x,y=document) => Array.from(y.querySelectorAll(x))

function get_filter_map(elt,sel,pred = x => true,trans = x => x) {
    if( typeof elt == "string" ) elt = q(elt)
    return qq(sel,elt).filter(pred).map(trans)
}
function get_matching(elt,pred = x => true) {
    return get_filter_map(elt,"a",x => pred(x.href),x => x.href)
}
```