code: def get_safe_variables(self,query,var_dict):
          ''' Create a list of safe variables: variables bounded to constants or
          that occur in a positive goal '''
  
          variables_bounded_to_constants = []
  
          for constraint in query.constraints:
              if (constraint.get_left_side().is_variable() and
                  constraint.get_right_side().is_constant() and
                  constraint.is_equality_constraint()):
                      variables_bounded_to_constants.append(constraint.get_left_side())
  
          safe_variables = list(var_dict) + variables_bounded_to_constants
  
          return safe_variablef get_safe_variables(self,query,var_dict):
          ''' Create a list of safe variables: variables bounded to constants or
          that occur in a positive goal '''
  
          variables_bounded_to_constants = []
  
          for constraint in query.constraints:
              if (constraint.get_left_side().is_variable() and
                  constraint.get_right_side().is_constant() and
                  constraint.is_equality_constraint()):
                      variables_bounded_to_constants.append(constraint.get_left_side())
  
          safe_variables = list(var_dict) + variables_bounded_to_constants
  
          return safe_variables
docs: Create a list of safe variables: variables bounded to constants or
          that occur in a positive goal