Fact can only be modified once based on the fact name #17

Hi,

I would like for my rule engine to run, save some values and then use those values, after the run, to modify some facts. For background: I want to run the engine 'dynamically': for each time step, I am running the engine, it provides a result, and based on that my facts change for the next time instance (the next run).

A minimal working example (without any relevant facts and rules):

import experta as exp
   
class Fact_1(exp.Fact):
    value_1 = exp.Field(int)

class Fact_2(exp.Fact):
    value_2 = exp.Field(int)
    
class Rules(exp.KnowledgeEngine):
    @exp.Rule(Fact_1(value_1 = exp.MATCH.v1),
              Fact_2(value_2 = exp.MATCH.v2))
    def subtract(self, v1, v2):
        return v1-v2
    
    
engine = Rules()
fact_1 = Fact_1(value_1 = 1)
fact_2 = Fact_2(value_2 = 3)
engine.reset()
engine.declare(fact_1)
engine.declare(fact_2)

engine.run()
engine.modify(fact_1)

engine.run()
engine.modify(fact_1)

This yields a Fact not founderror, I think because any time a fact is modified, it moves to the end of the fact list (engine.facts), while it is the number in this list that is used for the next modification.

I am new to working with rule-engines, so this might be a very specific problem that only I am facing, but I still have the feeling this might be a useful thing to have (i.e. repeated modification of facts by use of their name).