code: def insert_header(self, hkey, value, index=None):
          
          #if hkey is '': return
  
          # if it's an integer, use the hkey from the list
          if type(hkey) in [int, int]: hkey = self.hkeys[hkey]
  
          # set the data
          self.headers[str(hkey)] = value
          if not hkey in self.hkeys:
              if index is None: self.hkeys.append(str(hkey))
              else:             self.hkeys.insert(index, str(hkey))
          return self
docs: This will insert/overwrite a value to the header and hkeys.
  
          Parameters
          ----------
          hkey
              Header key. Will be appended to self.hkeys if non existent, or 
              inserted at the specified index. 
              If hkey is an integer, uses self.hkeys[hkey].
          value
              Value of the header.
          index=None
              If specified (integer), hkey will be inserted at this location in 
              self.hkeys.