I do not know. I tend to agree `isTRUE()` is better since it is safer. There are always holes like this in R, where we need to _assume_ users do not do stupid things like passing a character string to an argument that expects a logical value. In this case, a default value `enable = TRUE` may help users recognize that `enable` expects a logical scalar.

I just wanted to say `xor`() was easier for me to understand the condition here. I'm not good at turning my head upside down (`!conditionA`) and then looking at the other side (`conditionB`) :)