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Solve almost block-diagonal linear system

`x = slvblk(blokmat,b) x = slvblk(blockmat,b,w) `

`x = slvblk(blokmat,b) ` returns
the solution (if any) of the linear system `Ax = b`,
with the matrix `A` stored in `blokmat` in
the spline almost block-diagonal form. At present, only the command `spcol` provides
such a description, of the matrix whose typical entry is the value
of some derivative (including the 0th derivative, i.e., the value)
of a B-spline at some site. If the linear system is overdetermined
(i.e., has more equations than unknowns but is of full rank), then
the least-squares solution is returned.

The right side `b` may contain several columns,
and is expected to contain as many rows as there are rows in the matrix
described by `blokmat`.

`x = slvblk(blockmat,b,w) `
returns the vector `x` that minimizes the *weighted* sum
Σ_{j}*w*(*j*)((*Ax* – *b*)(*j*))^{2}.

`sp=spmak(knots,slvblk(spcol(knots,k,x,1),y.'))` provides
in `sp` the B-form of the spline s of order `k` with
knot sequence `knots` that matches the given data `(x,y)`,
i.e., for which s`(x)` equals `y`.

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