# File lib/spreadsheet/excel/writer/workbook.rb, line 96
  def font_index workbook, font_key
    idx = @fonts[workbook][font_key] || 0
    ## this appears to be undocumented: the first 4 fonts seem to be accessed
    #  with a 0-based index, but all subsequent font indices are 1-based.
    idx > 3 ? idx.next : idx
  end