Dollar-quoted string literals: 'none', 'untagged' ($$…$$ only) or 'tagged' ($$…$$ and $tag$…$tag$).
'none'
'untagged'
$$…$$
'tagged'
$tag$…$tag$
Dollar-quoted string literals:
'none','untagged'($$…$$only) or'tagged'($$…$$and$tag$…$tag$).