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opSpot/lsqr
 LSQR   LSQR Method.
 
    X = lsqr(A,B) attempts to solve the least-squares problem
 
        minimize  || A*X - B ||_2
 
    This routine is simply a wrapper to Matlab's own LSQR routine,
    and the argument list variations described in Matlab's LSQR
    documentation are also allowed here.  The usage is identical to
    Matlab's default version, except that the first argument must be a
    Spot operator.