code: def fill_field(self, ypos, xpos, tosend, length):
          
          if length < len(tosend):
              raise FieldTruncateError('length limit %d, but got "%s"' % (length, tosend))
          if xpos is not None and ypos is not None:
              self.move_to(ypos, xpos)
          self.delete_field()
          self.send_string(tosend)
docs: clears the field at the position given and inserts the string
              `tosend`
  
              tosend: the string to insert
              length: the length of the field
  
              Co-ordinates are 1 based, as listed in the status area of the
              terminal.
  
              raises: FieldTruncateError if `tosend` is longer than
                  `length`.