svoid test_TypeSystem() {
  GOperator<Class> iround = new GOperator<Class>("iround").arg(double.class).returns(int.class);
  GOperator<Class> plus_int = new GOperator<Class>("plus_int").arg(int.class).arg(int.class).returns(int.class);
  GOperator<Class> plus_dbl = new GOperator<Class>("plus_dbl").arg(double.class).arg(double.class).returns(double.class);
  print(iround);
  print(plus_int);
  print(plus_dbl);
  
  GPolymorphicOperator<Class> plus = new("+", plus_int, plus_dbl);
  print(plus.toStringWithDefinitions());
  
  new TypeSystem<Class> ts;
  assertTrueVerbose(ts.operatorCanTakeArguments(plus_dbl, double.class, double.class));
  assertFalseVerbose(ts.operatorCanTakeArguments(plus_dbl, double.class, int.class));
  assertFalseVerbose(ts.operatorCanTakeArguments(plus_dbl, int.class, double.class));
  assertFalseVerbose(ts.operatorCanTakeArguments(plus_dbl, double.class));
  assertFalseVerbose(ts.operatorCanTakeArguments(plus_dbl, double.class, double.class, double.class));
  
  assertEqualsVerbose(ll(plus_int), ts.resolve(plus, int.class, int.class));
  assertEqualsVerbose(ll(plus_dbl), ts.resolve(plus, double.class, double.class));
  
  // can't resolve because we haven't defined any subtype relations so int and double are incompatible
  assertEqualsVerbose(ll(), ts.resolve(plus, int.class, double.class));
  
  // Now let's test subtypes. Let's say an int is convertible to a double.
  ts.isActualSupertypeOf = (a, b) -> a == double.class && b == int.class;
  
  assertTrueVerbose(ts.operatorCanTakeArguments(plus_int, int.class, int.class));
  assertFalseVerbose(ts.operatorCanTakeArguments(plus_int, double.class, int.class));
  assertTrueVerbose(ts.operatorCanTakeArguments(plus_dbl, int.class, int.class));
  assertTrueVerbose(ts.operatorCanTakeArguments(plus_dbl, double.class, int.class));
}