Settable and testable rule flags

[david-pfx]

1. Rule flags are identifiers starting with '$': $winning. Just use them, no need to declare them.
2. They are set or cleared by a command: [ player | shark ] -> set $indanger
3. They are tested as a rule prefix: $winning [ player ] -> [ happyplayer ]. If false, the rule is skipped.
4. Flags are cleared at the start of each level. Flags with double '$' are not: $$wonlevel3.
5. Boolean expressions like $winning and not $indanger would be nice too.