find p l returns Some x where x is the first element of l such that p x returns true or None if such element as not been found.

rfind p l returns Some x where x is the last element of l such that p x returns true or None if such element as not been found.

findi p e l returns Some (i, ai) where ai and i are respectively the first element of l and its index, such that p i ai is true, or None if no such element has been found.

rfindi p e l returns Some (i, ai) where ai and i are respectively the last element of l and its index, such that p i ai is true, or None if no such element has been found.

Whenever n is inside of l size bounds, split_at n l returns `Ok (l1,l2), where l1 contains the first n elements of l and l2 contains the others. Otherwise, returns `Invalid_index n

If n is inside the bounds of l, at l n returns `Ok x, where x is the n-th element of the list l. Otherwise, returns `Invalid_index n.

assoc a l returns Some b where b is the value associated with key a in the list of pairs l. That is, assoc a [ ...; (a,b); ...] = Some b if (a,b) is the leftmost binding of a in list l. Return None if there is no value associated with a in the list l.

As assoc but with physical equality