!7

replace SimpleStatements with MatchableRecords<Prop>.
replace Env with SimpleStatements.

abstract sclass Prop implements IFieldsToList {}

static withToList record Did$HappenBefore$<A, B>(A event1, B event2) {}
static withToList record WhenDid$Happen(O event) extends Prop {}
static withToList record $HappensAtSecond$<A, B>(A event, B y) extends Prop {}

static O answer(Env env, O p) {
  if p is WhenDid$Happen(O event)
    ret env.fulfillVariable(var -> $HappensAtSecond$(event, var));

  if p is Did$HappenBefore$(O event1, O event2) once {
    Int time1 = opt cast answer(env, WhenDid$Happen(event1));
    if (time1 == null) break;
    Int time2 = opt cast answer(env, WhenDid$Happen(event2));
    if (time2 == null) break;
    ret time1 < time2;
  }
  
  null;
}

p-exp {
  SimpleStatements env = new(
    $HappensAtSecond$("stuff", 10),
    $HappensAtSecond$("more stuff", 20));

  assertEqualsVerbose(true, answer(env, print(Did$HappenBefore$("stuff", "more stuff"))));
  assertEqualsVerbose(false, answer(env, print(Did$HappenBefore$("more stuff", "stuff"))));
  assertEqualsVerbose(null, answer(env, print(Did$HappenBefore$("stuff", "any other stuff"))));
}