--- first?
#    is a predicate to test the call is a first call to first? with
#    current group.
#    It is useful to format comma separated values as:
    ߤΥ롼פ first? ФǽθƤӽФɤȽꤹ
    ҸǤ
    ϥޤǶڤ줿ͤΤͭѤǤ

      pp.group(1, '[', ']') {
        xxx.each {|yyy|
          unless pp.first?
            pp.text ','
            pp.breakable
          end
          ... pretty printing yyy ...
        }
      }

