docs: def read_excel(filename, sheet=None):
      """Reads an Excel file containing a tabular description of a transition function,
         as found in Sipser. Major difference: instead of multiple header rows,
         only a single header row whose entries might be tuples.
         """
  
      from openpyxl import load_workbook
      wb = load_workbook(filename)
      if sheet is None:
          ws = wb.active
      else:
          ws = wb.get_sheet_by_name(sheet)
      table = [[cell.value or "" for cell in row] for row in ws.rows]
      return from_table(table)