code: def decompose_code(code):
      
      code = "%-6s" % code
      ind1 = code[3:4]
      if ind1 == " ": ind1 = "_"
      ind2 = code[4:5]
      if ind2 == " ": ind2 = "_"
      subcode = code[5:6]
      if subcode == " ": subcode = None
      return (code[0:3], ind1, ind2, subcode)
docs: Decomposes a MARC "code" into tag, ind1, ind2, subcode