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