docs: def _add_constraint(self, constraint):
          """
          Adds constraint to the ProbModelXML.
          """
          constraint_data = self.data['probnet']['AdditionalConstraints'][constraint]
          constraint_element = etree.SubElement(
              self.additional_constraints, 'Constraint', attrib={'name': constraint})
          for argument in sorted(constraint_data):
              name = argument
              value = constraint_data[name]
              etree.SubElement(constraint_element, 'Argument', attrib={'name': name, 'value': value})