declaration/definition
group group-name (& group-name)* rule*
Indicates that the rules following this declaration are part of the named groups. More than one group declaration can name the same group: the rules following every group declaration for a given group name are members of the named group.
The following variations are permitted:
and can be used in place of "&".
Declarations and calling functions are not affected by groups. The selection of an error rule (markup-error or sgml-error) is also unaffected by which groups are currently active.