The type for prompt histories.

v ~prev initializes the toplevel with previous entries prev (later elements are older).

empty is an empty history.

entries h are all the entries in the history.

add h e makes adds entry v to history.

restart

prev h current makes current the next entry of the resulting history and returns the previous entry of h (if any).

next h current makes current the previous entry of the resulting history and returns the next entry of h (if any).

to_string ~sep t is a string with the entries of t separated by lines that contain sep.

of_string ~sep s is history from s assumed to be entries seperated by lines that contain sep.