The insert function takes an element and a list
and inserts the element into the list at the first position where it
is less than or equal to the next element. In particular, if the list
is sorted before the call, the result will also be sorted. It is a
special case of insertBy, which allows the programmer to supply
their own comparison function.
insert x xs inserts x into the last position
in xs where it is still less than or equal to the next
element. In particular, if the list is sorted beforehand, the result
will also be sorted.