I didn't notice that your version could check against subtrees. Another possibility would be to use errsafe along with the function (but I'm worried that might be too dangerous). Other than that I would have to agree with your design. I played around with the common lisp design of subst-if and found it does the exact same thing as yours but it also does not test nil. This can easily be done in arc in a similar way to atom&... with idfn&... I would have to go with your design for now but we might want to think about other designs such as whether or not to test nil.
