We can [...] reformulate the Alternation Condition as restricting the corresponding class of rules as follows: (16) "Revised Alternation Condition (RAC)" Obligatory neutralization rules apply only in derived environments. That is, an obligatory neutralization rule can apply only if the input involves crucially a sequence which arises in morpheme combinations or through the earlier application of a phonological rule. Otherwise, that is if the environment is met already in the underlying representation of a single morpheme, the process cannot apply.
- Kiparsky (2004), Pag. 133