These give parsed information about the current thing being
parsed, e.g. the current tag, container or entity. They return
zero if they're not applicable.
tag_name gives the name of the current tag. If used
from an entity callback, it gives the string inside the
entity.
tag_args gives the arguments of the current tag,
parsed to a convenient mapping consisting of key:value pairs.
If the current thing isn't a tag, it gives zero. default_value
is used for arguments which have no value in the tag. If
default_value isn't given, the value is set to the same string
as the key.
tag_content gives the content of the current tag, if
it's a container or quote tag.
tag() gives the equivalent of
({tag_name(),tag_args(), tag_content()}).