code: def is_label(token):
      
      if not LABELS_ALLOWED:
          return False
  
      c = i = token.lexpos
      input = token.lexer.lexdata
      c -= 1
      while c > 0 and input[c] in (' ', '\t'):
          c -= 1
  
      while i > 0:
          if input[i] == '\n':
              break
          i -= 1
  
      column = c - i
  
      if column == 0:
          column += 1
  
      return column == 1
docs: Return whether the token is a label (an integer number or id
      at the beginning of a line.
  
      To do so, we compute find_column() and moves back to the beginning
      of the line if previous chars are spaces or tabs. If column 0 is
      reached, it's a label.