|You can consider these as part of Pliant parser engine machinery, it is unlikely
that someone will write extra ones.
A good example is 'perse_filter_execute': it checks that the parser reached
a 'stable' state (parsed all a top level bloc), and start execution.
Without this parser, execution would only start at the end of the module
parsing, and it would be bad, because if you write a line such as
you expect that you can use operators defined in 'foo.pli' on the next line,
which is possible only if the 'module' instruction has been compiled/executed
in the mean time.