Implements eq/ne/hash via a _value_equalityvalues method.

_value_equalityvalues is a method that the decorated class must implement.

_value_equality_approximatevalues is a method that the decorated class might implement if special support for approximate equality is required. This is only used when approximate argument is set. When approximate argument is set and _value_equality_approximatevalues is not defined, _value_equalityvalues values are used for approximate equality. For example, this can be used to compare periodic values like angles: the angle value can be wrapped with PeriodicValue. When returned as part of approximate values a special normalization will be done automatically to guarantee correctness.

Note that the type of the decorated value is included as part of the value equality values. This is so that completely separate classes with identical equality values (e.g. a Point2D and a Vector2D) don't compare as equal. Further note that this means that child types of the decorated type will be considered equal to each other, though this behavior can be changed via the 'distinct_child_typesargument. The type logic is implemented behind the scenes by a_value_equality_valuescls` method added to the class.

cls The type to decorate. Automatically passed in by python when using the @cirq.value_equality decorator notation on a class.
unhashable When set, the hash method will be set to None instead of to a hash of the equality class and equality values. Useful for mutable types such as dictionaries.
distinct_child_types When set, classes that inherit from the decorated class will not be considered equal to it. Also, different child classes will not be considered equal to each other. Useful for when the decorated class is an abstract class or trait that is helping to define equality for many conceptually distinct concrete classes.
manual_cls When set, the method '_value_equality_valuescls' must be implemented. This allows a new class to compare as equal to another existing class that is also using value equality, by having the new class return the existing class' type. Incompatible with distinct_child_types.
approximate When set, the decorated class will be enhanced with _approx_eq_ implementation and thus start to support the SupportsApproximateEquality protocol.

TypeError If the class decorated does not implement the required _value_equality_values method or, if manual_cls is True, the class does not implement _value_equality_values_cls_.
ValueError If both distinct_child_types and manual_cls are specified.