code: def FindHeader(self, header):
      
      for section_list in self.include_list:
        for f in section_list:
          if f[0] == header:
            return f[1]
      return -1
docs: Check if a header has already been included.
  
      Args:
        header: header to check.
      Returns:
        Line number of previous occurrence, or -1 if the header has not
        been seen before.