Returns a subset of the current
ImmMap starting from a given key
location up to, but not including, the element at the provided length from
the starting key location
public function slice( int $start, int $len, ): ImmMap<Tk, Tv>;
$start is 0-based.
$len is 1-based. So
slice(0, 2) would return the
keys and values at key location 0 and 1.
int $start- The starting key location of the current
ImmMapfor the returned
int $len- The length of the returned