Options are neither mutual exclusive nor given precedence

[vanHoesel]

One can easily add the options add multiple options, which might be weird to do so, I would agree. But it is not documented what order will be used.

Should we either check for multiple options that should be mutual exclusively set?
Should we allow multiple options and make env_key to be the final resort, being explicitly set ?

I tend to go for the first solution, because it is the most easy. But I was about to start on #2, where I need multiple ENV-vars to be an option, to be able to deprecate without breaking people their stuff.

[mohawk2]

I think you would need to spell out with examples (which could end up in the doc) about what would happen in each case.