Adds all of the elements in the specified collection to this set if they're not already present. If the specified collection is also a set, the addAll operation effectively modifies this set so that its value is the union of the two sets. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
@param c collection containing elements to be added to this set
@return true if this set changed as a result of the call
@throws NullPointerException if the specified collection is null
@see #add(Object)
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