  We show that solutions to fermion sign problems in the CT-INT formulation can
be extended to systems involving fermions interacting with dynamical quantum
spins. While these sign problems seem unsolvable in the auxiliary field
approach, solutions emerge in the worldline representation of quantum spins.
Combining the idea with meron-cluster methods, we are able to extend the
solvable models even further. We demonstrate these novel solutions to sign
problems by considering several examples of strongly correlated systems.
