fopen ( ..., "a" ) --- how does the "a" work?

[RAMontante]

gwyn at brl.arpa (Doug Gwyn) <11775 at smoke.BRL.MIL> :
-On a single-user non-multitasking system, a better implementation
-[of append] would
-be to seek to the end only on the initial open, not for each write.

Is the process forbidden from doing an lseek, or are you allowing the
programmer to reposition somewhere else in the file?  What is the
semantics of the append behavior?