docs: def setdbo(self, bond1, bond2, dboval):
          """Set the double bond orientation for bond1 and bond2
          based on this bond"""
          # this bond must be a double bond
          if self.bondtype != 2:
              raise FrownsError("To set double bond order, center bond must be double!")
          assert dboval in [DX_CHI_CIS, DX_CHI_TRANS, DX_CHI_NO_DBO], "bad dboval value"
          
          self.dbo.append(bond1, bond2, dboval)