Which units are considered "flat-chested"?

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There are at least 2 game mechanics I know which only apply to "flat-chested" units (the Flatty Squad and the Anti-Flat Chest evility). Looking around the Internet I found a few examples, interesting anomalies and educated guesses, but no complete list of all classes and unique characters which are considered "flat-chested" in the context of these mechanics.
I am playing the PC version, if it matters.
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