\ Backslash or left matrix divide.
X = A\B is similar to Matlab's backslash operator. If A is a Spot
operator and B is a vector, then in general X is computed as the solution to the
(*) minimize ||Ax - b||_2.
However, some Spot operators implement their own mldivide routines
that determine exactly how a solution to (*) is obtained. For
example, the orthogonal operators (e.g., opWavlet) obtain X via
If A is a scalar and B is a spot operator, then X = opFoG(1/A,B).
The least-squares problem (*) is solved using LSQR with default
parameters specified by spotparams.