  We define sutured Heegaard diagrams for null-homologous knots in 3-manifolds.
These diagrams are useful for computing the knot Floer homology at the top
filtration level. As an application, we give a formula for the knot Floer
homology of a Murasugi sum. Our result echoes Gabai's earlier works. We also
show that for so-called 'semifibred' satellite knots, the top filtration term
of the knot Floer homology is isomorphic to the counterpart of the companion.
