Move point to beginning of line (after prompt) or to the process mark.
The first time you use this command, it moves to the beginning of the line
(but after the prompt, if any). If you repeat it again immediately,
it moves point to the process mark.
The process mark separates the process output, along with input already sent,
from input that has not yet been sent. Ordinarily, the process mark
is at the beginning of the current input line; but if you have
used M-x comint-accumulate to send multiple lines at once,
the process mark is at the beginning of the accumulated input. (fn)