  We derive a forcing axiom from the conjunction of square and diamond, and
present a few applications, primary among them being the existence of
super-Souslin trees. It follows that for every uncountable cardinal $\lambda$,
if $\lambda^{++}$ is not a Mahlo cardinal in G\"odel's constructible universe,
then $2^\lambda = \lambda^+$ entails the existence of a $\lambda^+$-complete
$\lambda^{++}$-Souslin tree.
