Prepare the right-hand side for the linear system op(u)=rhs(f).
In matrix form, LDG looks like this:
Mv = Cu + g Mf = Av + Bu + h
where v is the auxiliary vector, u is the argument of the operator, f
is the result of the grad operator, g and h are inhom boundary data, and
A,B,C are some operator+lifting matrices.
M f = A Minv(Cu + g) + Bu + h
so the linear system looks like
M f = A Minv Cu + A Minv g + Bu + h M f - A Minv g - h = (A Minv C +
B)u (*) --------rhs-------
So the right hand side we're putting together here is really
M f - A Minv g - h
Finally, note that the operator application above implements the
equation (*) left-multiplied by Minv, so that the right-hand-side
becomes
f - Minv( A Minv g + h)
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