\typein[
cmd]{
msg}
\typein[\cmd]{msg}
\typein
prints msg on the terminal and causes LaTeX to
stop and wait for you to type a line of input, ending with return. If
the optional \cmd argument is omitted, the typed input is
processed as if it had been included in the input file in place of the
\typein
command. If the \cmd argument is present, it
must be a command name. This command name is then defined or
redefined to be the typed input.