the mutation is rare in old norse, e.g. "verr" "man", "heðan" "hence", "neðan" "from below" in contrast to "niðr" "down(wards)" and perhaps "jafn" "even". instances where "a"-mutation has failed to occur in old norse can mostly be explained as analogical forms, although a palatal stop or immediately preceding the in a short root syllable has a tendency to block or reverse the process.