E.g. `@` isn't defined on https://wiki.apertium.org/wiki/Apertium-recursive/Formalism#Tag_Rewrite_Rules similarly `$`, would be nice to have a `List of Symbols`.
E.g.
@isn't defined on https://wiki.apertium.org/wiki/Apertium-recursive/Formalism#Tag_Rewrite_Rulessimilarly
$, would be nice to have aList of Symbols.