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bbconst  Create a black-box operator of constants.
   bbconst(M) creates an M-by-M operator of ones
   bbconst(M,N) is M-by-N, and
   bbconst(M,N,C) and bbconst([M,N],C) creates
   an M-by-N operator where all elements is the constant C.
   See also bbeye, bbzeros.