Returns the greatest element in this set less than or equal to
the given element, or {@code null} if there is no such element.
@param e the value to match
@return the greatest element less than or equal to {@code e},
or {@code null} if there is no such element
@throws ClassCastException if the specified element cannot be
compared with the elements currently in the set
@throws NullPointerException if the specified element is null
and this set does not permit null elements
Returns the greatest element in this set less than or equal to the given element, or {@code null} if there is no such element.
@param e the value to match @return the greatest element less than or equal to {@code e}, or {@code null} if there is no such element @throws ClassCastException if the specified element cannot be compared with the elements currently in the set @throws NullPointerException if the specified element is null and this set does not permit null elements