Synchronic and diachronic approaches can reach quite different conclusions. For example, a Germanic strong verb like English "sing – sang – sung" is irregular when it is viewed synchronically: the native speaker's brain processes them as learned forms, but the derived forms of regular verbs are processed quite differently, by the application of productive rules (for example, adding "-ed" to the basic form of a verb as in "walk – walked"). That is an insight of psycholinguistics, which is relevant also for language didactics, both of which are synchronic disciplines. However, a diachronic analysis shows that the strong verb is the remnant of a fully regular system of internal vowel changes, in this case the Indo-European ablaut; historical linguistics seldom uses the category "irregular verb".