/usr/lib/swipl/library/ext/clib/process.pl
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 prolog_executable(+Tool, -Exe, +ArgvIn, -Argv) is det[multifile, library(process)]
 prolog:prolog_tool(+Tool, -Exe, +ArgvIn, -Argv) is semidet[multifile, library(process)]
Find the executable and commandline arguments for running Tool. This provides a hook for process_create/3 called as process_create(prolog(Tool), ...). Tool is currently one of:
[115, 101, 108, 102]
Run Prolog itself.
[115, 119, 105, 112, 108]
Run the commandline version, also when called from the swipl-win app.
[115, 119, 105, 112, 108, 45, 119, 105, 110]
Run the swipl-win app, also when called from the commandline version.
[115, 119, 105, 112, 108, 45, 108, 100]
Run the C/C++ compiler frontend to embed Prolog or build foreign extensions.

prolog:prolog_tool/4 is defined as multifile and dynamic and can be used for special cases. This hook is notably intended to provide a portable way of calling Prolog when Prolog is embedded.

For example, when using rolog, we can run Prolog using

R -s -e 'rswipl::swipl()', '—args' <Prolog Argv>

We can make process_create(prolog(swipl), ...) work using

:- multifile prolog:prolog_tool/4.
prolog:prolog_tool(swipl, path('R'), Argv,
                   [ '-s', '-e', 'rswipl::swipl()', '--args'
                   | Argv
                   ]).