Any way to make a Mac a wireless access point, wirelessly?

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I know you can share a Mac's wired connection easily as a wireless hotspot, but can it be done wirelessly?

I was just reading that one can do this on Windows with a single wireless card. In other words, they receive the wireless signal with a wireless card and then share it out using the same - single antenna - card.

An example of this would be at an airport where one person purchases a wireless connection, then can share it out to others wirelessly.