declaration/definition
group group-name (& group-name)*
   rule*
    
 group 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 &, and
          
 Declarations and 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.