!7

replace Var with ValuelessVar.
replace VarMap with Map<Var, O>.
replace SimpleStatements with MatchableRecords<Prop>.

abstract sclass Prop implements IFieldsToList {}

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

static O answer(O p, SimpleStatements statements) {
  if p is WhenDid$Happen(O event) {
    ret statements.fulfillVariable(var -> $HappensAtSecond$(event, var));
  }
  null;
}

p-exp {
  SimpleStatements statements = new(
    $HappensAtSecond$("stuff", 10),
    $HappensAtSecond$("more stuff", 20)
  );
  assertEqualsVerbose(10, answer(print(WhenDid$Happen("stuff")), statements));
  assertEqualsVerbose(20, answer(print(WhenDid$Happen("more stuff")), statements));
  assertEqualsVerbose(null, answer(print(WhenDid$Happen("something else")), statements));
}