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bbhadamard  Create a black-box operator for a Hadamard matrix.
   BB=bbhadamard(N) creates an N-by-N Hadamard matrix. A Hadamard matrix
   is a unitary matrix where all elements are 1 or -1.

   For N>2 it is necessary that rem(N,4)=0 for a Hadamard matrix to exist.
   It is conjectured that this is also sufficient. bbhadamard requires that
   N can be factorized as a product of 2, 12, 20, 28, 36, and 44.

   An operator near the requested size can be retrieved as follows:
     [BB,N2]=bbhadamard(N,'next')   Ensure N2>=N
     [BB,N2]=bbhadamard(N,'prev')   Ensure N2<=N

   N2 is the order of the generated operator.

   See also .