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A Lindenmayer System invented by Hilbert 
 (1891) whose limit is a Plane-Filling Curve which fills a
square.  Traversing the Vertices of an 
-D Hypercube in Gray Code order produces
a generator for the 
-D Hilbert curve (Goetz).  The Hilbert curve can be simply encoded with initial string "L",
String Rewriting rules "L" -> "+RF-LFL-FR+", "R"->"-LF+RFR+FL-", and angle 90° (Peitgen and Saupe 1988,
p. 278).
A related curve is the Hilbert II curve, shown above (Peitgen and Saupe 1988, p. 284). It is also a Lindenmayer System and the curve can be encoded with initial string "X", String Rewriting rules "X" -> "XFYFX+F+YFXFY-F-XFYFX", "Y" -> "YFXFY-F-XFYFX+F+YFXFY", and angle 90°.
See also Lindenmayer System, Peano Curve, Plane-Filling Curve, Sierpinski Curve, Space-Filling Curve
References
Bogomolny, A.  ``Plane Filling Curves.''  
http://www.cut-the-knot.com/do_you_know/hilbert.html.
 
Dickau, R. M.  ``Two-Dimensional L-Systems.''
http://forum.swarthmore.edu/advanced/robertd/lsys2d.html.
 
Dickau, R. M.  ``Three-Dimensional L-Systems.''
http://forum.swarthmore.edu/advanced/robertd/lsys3d.html.
 
Goetz, P.  ``Phil's Good Enough Complexity Dictionary.''
  http://www.cs.buffalo.edu/~goetz/dict.html.
 
Hilbert, D.  ``Über die stetige Abbildung einer Linie auf ein Flachenstück.''  Math. Ann. 38, 459-460, 1891.
 
Peitgen, H.-O. and Saupe, D. (Eds.).  The Science of Fractal Images.  New York: Springer-Verlag, pp. 278 and 284, 1988.
 
Wagon, S.  Mathematica in Action.  New York: W. H. Freeman, pp. 198-206, 1991.