code: def write_mRNA_children(self, db, mRNA_id):
          
          mRNA_children = db.children(mRNA_id, order_by='start')
          nonexonic_children = []
          for child_rec in mRNA_children:
              if child_rec.featuretype == "exon":
                  self.write_rec(child_rec)
                  self.write_exon_children(db, child_rec)
              else:
                  nonexonic_children.append(child_rec)
          self.write_recs(nonexonic_children)
docs: Write out the children records of the mRNA given by the ID
          (not including the mRNA record itself) in a canonical
          order, where exons are sorted by start position and given
          first.