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A Sequence which arises in the hypothetical reproduction of a population of rabbits.  Let the Substitution
Map 
 correspond to young rabbits growing old, and 
 correspond to old rabbits producing young rabbits.
Starting with 0 and iterating using String Rewriting gives the terms 1, 10, 101, 10110, 10110101, 1011010110110,
....  The limiting sequence written as a Binary Fraction 
 (Sloane's A005614), where
 denotes a Binary number (a number in base-2) is called the Rabbit Constant.
See also Rabbit Constant, Thue-Morse Sequence
References
Schroeder, M.  Fractals, Chaos, Power Laws: Minutes from an Infinite Paradise.  New York: W. H. Freeman, p. 55, 1991.
 
Sloane, N. J. A.  Sequence 
A005614
in ``The On-Line Version of the Encyclopedia of Integer Sequences.''
http://www.research.att.com/~njas/sequences/eisonline.html.