class RubyXL::Workbook

def add_worksheet(name = nil)
  if name.nil? then
    n = 0
    begin
      name = SHEET_NAME_TEMPLATE % (n += 1)
    end until self[name].nil?
  end
  new_worksheet = Worksheet.new(:workbook => self, :sheet_name => name)
  worksheets << new_worksheet
  new_worksheet
end