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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
