pengines_sandbox.pl -- Declare Pengine interaction sandbox-safe

This module can be loaded alongside library(pengines) to allow for calling remote pengines from a sandboxed environment. This is disallowed by default because one of the use-cases of sandboxed pengines is to provide a generic application interface. In such cases you do not want the application to act as a proxy, in particular not to other systems in a protected network.

 sandbox:safe_primitive(+Goal) is semidet[multifile]

Declare the core pengine operations as safe. If we are talking about local pengines, their safety is guaranteed by the sandboxing done for all pengines.

To be done

- If at some point we allow for `unsafe' pengines, we must reconsider this.

 sandbox:safe_meta(+Goal, -Called) is semidet[multifile]

Declare the pengine meta-predicates as safe. Note that the pengine calling predicates are safe because the safety is guaranteed by the recieving pengine.