* Mutually exclusive css classes: I don't think you need it. Most of it can be covered by the order of css rules in your css files, or by specifity, and ultimately: `classnames` allows the on/off switching of classes controled by property values or other conditions and I have not seen any parts of your current project where that would not be sufficient: So with classname you can do something like:
`<div classname={classnames({ 'foo_is-active': someProp, 'foo_is-deactivated': !someProp })} />`