init l inits a focused list. It raises an exception Empty_list if l is empty, hence has no element to focus on.

forward z returns a the new focused_list where the focused element is the step steps (default to 1) next to the current one in the initial list. It raises End_of_list if no such element is available

backward z returns a the new focused_list where the focuses element is the previous one in the initial list. It raises End_of_list if no such element is available

fold f a z returns f (... (f (f a z1) z2) ...) z_n where z_1=z and z_{i+1}=forward z_i and forward z_n would raise an exception End_of_list.

fold_forward ~include_focus f a z returns f (... (f (f a z1) z2) ...) z_n where z_1=focus z if include_focus is true (default) and z_1=focus (forward z) otherwise (returns a if forward z raises End_of_list) and z_{i+1}=focus (forward z_i) and forward z_n would raise an exception End_of_list.

focus l returns the focus element of the focused list l