// @dart = 2.9
class Complex {
  final double real;
  final double imaginary;
  const Complex(this.real, this.imaginary);
  Complex add(Complex other) {}
  Complex sub(Complex other) {}
  Complex negate() {}
  int get hashCode => real.hashCode * 13 + imaginary.hashCode * 19;
  bool operator ==(Object other) {}
  String toString() => 'Complex($real,$imaginary)';
}

extension Operators on Complex {
  Complex operator +(Complex other) => add(other);
  Complex operator -(Complex other) => sub(other);
  Complex operator -() => negate();
}

main() {}
implicit() {}
explicit() {}
void errors(Complex c) {}
expect(expected, actual) {}