code: public void keys(final IntArrayList list) {
  	list.clear();
  	forEachKey(
  		new IntProcedure() {
  			public boolean apply(int key) {
  				list.add(key);
  				return true;
  			}
  		}
  	);
  }
docs: Fills all keys contained in the receiver into the specified list.
  Fills the list, starting at index 0.
  After this call returns the specified list has a new size that equals <tt>this.size()</tt>.
  Iteration order is guaranteed to be <i>identical</i> to the order used by method {@link #forEachKey(IntProcedure)}.
  <p>
  This method can be used to iterate over the keys of the receiver.
  
  @param list the list to be filled, can have any size.