docs: def strip_HETATMs(self, only_strip_these_chains = []):
          '''Throw away all HETATM lines. If only_strip_these_chains is specified then only strip HETATMs lines for those chains.'''
          if only_strip_these_chains:
              self.lines = [l for l in self.lines if not(l.startswith('HETATM')) or l[21] not in only_strip_these_chains]
          else:
              self.lines = [l for l in self.lines if not(l.startswith('HETATM'))]
          self._update_structure_lines()