on
A hook invoked when stopIterations() is called, before the solver's own iterative process is signaled to stop. The stop flag of the iterative process is only checked between outer iterations, so a composite solver that blocks inside an iteration waiting on inner solvers (possibly running on other threads) overrides this to forward the stop request to those in-flight inner solvers. The default does nothing.
Parameters
msg
the message supplied to stopIterations, possibly null