Function wrapper for applying DLP preconditioners.
SYNOPSIS
function t = precDLP(D,L,P,Bx,s)
DESCRIPTION
Function wrapper for applying DLP preconditioners.
t = precDLP(D,L,P,Bx,s)
solves the equation
P_{Bx,Bx} A P_{Bx,Bx} t = s
with solution
t = P_{inv(A)Bx,Bx} inv(A) s
using an approximate inverse of A,
P A P^T ~= L D L^T
It is assumed that P_{Bx,Bx} s = s, i.e.
s is perpendicular to Bx
