Adds the specified element to this set if it is not already present.
More formally, adds the specified element <tt>e</tt> to this set if
this set contains no element <tt>e2</tt> such that
<tt>(e is null ? e2 is null : e.equals(e2))</tt>.
If this set already contains the element, the call leaves the set
unchanged and returns <tt>false</tt>.
@param e element to be added to this set
@return <tt>true</tt> if this set did not already contain the specified
element
Adds the specified element to this set if it is not already present. More formally, adds the specified element <tt>e</tt> to this set if this set contains no element <tt>e2</tt> such that <tt>(e is null ? e2 is null : e.equals(e2))</tt>. If this set already contains the element, the call leaves the set unchanged and returns <tt>false</tt>.
@param e element to be added to this set @return <tt>true</tt> if this set did not already contain the specified element