This symbol is returned by next to signal that iteration is complete.
i1 : iter = iterator {1, 2, 3} o1 = iterator {1, 2, 3} o1 : Iterator
i2 : next iter o2 = 1
i3 : next iter o3 = 2
i4 : next iter o4 = 3
i5 : next iter o5 = StopIteration o5 : Symbol
The object StopIteration is a symbol.