hese changes plumb basic support for cancellation through the engine.
Two types of cancellation are supported for all engine operations:
- Cancellation, which waits for the operation to drive itself to a safe
point before the operation returns, and
- Termination, which does not wait for the operation to drive itself
to a safe opint for the operation returns.
When updating local or managed stacks, a single ^C triggers cancellation
of any running operation; a second ^C will trigger termination.
Fixes#513, #1077.
