  The magnetization $M(h)$ is used to identify three singlet quantum phases of
the ladder with isotropic exchange interactions. The Dimer phase with
frustrated F exchanges in rungs and legs has a first-order $M(h)$ transition at
$0$ K from singlet to ferromagnetic at the saturation field $h_s$. The
Haldane-DAF phase with strong F exchange in rungs and net AF exchange between
rungs has continuous $M(h)$ and is adiabatically connected to the $S = 1$
Heisenberg AF chain. The AF phase with strong F exchange in legs and net AF
exchange between legs has continuous $M(h)$ and is adiabatically connected to
the spin-1/2 $J_1-J_2$ model with $J_1 > 0$ and $J_2 < 0$. All three singlet
phases have finite gaps to the lowest triplet state.
