Found it in the readline manual https://tiswww.cwru.edu/php/chet/readline/readline.html#index-insert_002dcomment-_0028M_002d_0023_0029 https://tiswww.cwru.edu/

Found it in the readline manual https://tiswww.cwru.edu/php/chet/readline/readline.html#index-insert_002dcomment-_0028M_002d_0023_0029 https://tiswww.cwru.edu/php/chet/readline/readline.html#inde... insert-comment (M-#) Without a numeric argument, the value of the comment-begin variable is inserted at the beginning of the current line.