docs: def _emplace_pmrna(mrnas, parent, strict=False):
      """Retrieve the primary mRNA and discard all others."""
      mrnas.sort(key=lambda m: (m.cdslen, m.get_attribute('ID')))
      pmrna = mrnas.pop()
      if strict:
          parent.children = [pmrna]
      else:
          parent.children = [c for c in parent.children if c not in mrnas]