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rectangle: a list of lists of equal length

files: 
 mrlib/matrix-snip.ss     : the image snips for matrix 

 			    works with matrices that implement matrix<%>
			    i.e., support a ->rectangle method

			    it writes out a matrix as a rectangle and 
			    reconstructs it as a rectangle 

			    the function visible-matrix may therefore yield a
			    rectangle or a matrix representation proper

 drscheme/private/eval.ss : requires matrix-snip to share at module level 

 htdp/matrix.ss           : uses snips to present matrices, requires matrix-snip
 htdp/matrix-invisible.ss : make matrices invisible

 			  * they are created from two mutually recursive units: 
			  * matrix-unit and a 'rendering' unit 

 htdp/matrix-sig.ss	  : the functions that matrix-unit.ss provides 
 			    and that matrix-render.ss needs 

 htdp/matrix-render-sig.ss: the functions that matrix-unit expects from the 
 			    rendering unit

 htdp/matrix-unit.ss 	  : the matrix functionality 

 htdp/tests/matrix-test.ss: a textual test 
 htdp/tests/matrix-client.ss a test with embedded images 
