Discards some bytes from the internal buffer in an async-io-state.
The function async-io-state-discard discards the first discard bytes from the internal buffer in async-io-state. The rest of the buffer is preserved for future reading.
async-io-state-discard must only be called inside the scope of the callback of async-io-state-read-with-checking. Once async-io-state-discard has been called, the callback must not access the buffer again.
The return value unread-buffer-length is the remaining number of bytes in the buffer.
LispWorks User Guide and Reference Manual - 13 Feb 2015