We have a function and a piece of code that validates that one of specific parameters is present. Besides arguing whether we should state nullity or presence (e.g.
Only one is not null vs
Only one is present), we have a confusion about which word is more appropriate when it is used to state that there's only one conforms to a condition.
In other words, should we say
Only one not null or
Exactly one not null meaning that one parameter is present, but all others are absent?
Or maybe "Either is present" would be a perfect match? What do you think?