lusive ending index of the slice
          If negative:
            the exclusive ending index of the slice from
            the end of the vector.
          If NULL: same as the length of the vector
STEP  --- If above zero:
            the step size between elements of the slice
          If NULL: same as one

Note that this always modifies the passed vector, and
adjusts the fill pointer, if one is present.

See <a href="#FUNCTION%20ARRAY-UTILS%3ANSLICE" class="xref">NSLICE</a>
See <a href="#FUNCTION%20ARR