Isn't that only the case when the patents cover two different parts of your own implementation? How could someone patent something that has already been patented?

Let’s say you invent the cart, and in the patent, you describe a platform with two wheels and two handles to push it.

Then, I can patent a cart with “a number of wheels” and “at least one handle”.

That’s why patents tend to be so vague.

You don’t want to keep the tiniest door open for such cases, so you don’t use a mirror, you use a light-reflecting device. If you’re smart, you don’t even use the term ‘light’. Your invention likely works with radar and röntgen, too. Maybe avoid electromagnetic fields, too. Who knows whether something similar, in the abstract, is possible with sound?

You also don’t say a device needs to do A, then B if there’s a tiny chance it might be possible to accomplish the same doing B first, then A.

That's not really a good example since a cart with two wheels and a two handles anticipates under 35 U.S.C Sec. 102 a cart with a number of wheels and at least one handle.

But yea generally you want to describe things with a billion examples, but no statements limiting the scope of your invention.

The most devious strategy is that you disclose vague and probably impossible embodiment. Wait 10 years and then write claims that that narrowly target big products that you really couldn't have invented ten years. But the burden is more or less on the accused infringer to provide you didn't describe the new invention in the old patent.

This sounds horrible. Isn't the patent is meant to be practically usefull by a proffesional to build the device?

From my experience reading US patents/applications, they aren't just vague like that. They start out specific and then add additional claims which expand it. So they will have some claim about a cart with a specific number of wheels, but in later claims they'll add language to generalize so any number of wheels are covered. There are also figures and detailed descriptions which are separate from the claims which aim to make clearer. Pretty much any one I have read seems to make it clear enough create the device.

Team A creates a patent covering X. Team B licenses Team As patent and creates a patent covering X+A.

Turns out everyone who does X also needs to do A, to the extent X is X+A, so everyone needs to license both patents.

Makes sense but Team B's patent would be for their work, not for Team A's work, right? I get that you'd need to license both in this scenario, but the actual tech claimed by the patents wouldn't overlap.