It's just a shortcut for the & operator, like += or *=.
Code:
a = b + c
If a is b, then you can see that
a = a + c
If that is the equation, then you can shortcut
a += c
So that line is really doing
Code:
iButtons = iButtons & ~BUTTONS_REMOVE;
& is a bitwise operator.
For more information on those, check out the Code Snippets/Tutorials section.
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