  Many of the ideas in this entry may be illustrated using streams.  A stream of numbers is an ordered pair whose first coordinate is a number and whose second coordinate is again a stream of numbers.  The first coordinate is called the head, and the second the tail.  The tail of a given stream might be different from it, but again, it might be the very same stream.  For example, consider the stream s whose head is 0 and whose tail is s again.  Thus the tail of the tail of s is s itself.  We have s = ⟨0, s⟩ ,   s =  ⟨0, ⟨0, s⟩  ⟩ ,   etc. This stream s exhibits object circularity.  It is natural to “unravel” its definition as: