(particle equivalent to 啊 after noun ending in -n)
sentence-final particle (abbr. for 呢啊[ne5 a5] or variant of 哪[na5])
(adverbial expression indicating that the attention of the subject of the verb is focused on what they are doing, not distracted by anything else)
just ...ing (and nothing else)
Donald Knuth (1938-), famous American computer scientist at Stanford University
For goodness sake!