default: Any = cirq.protocols.measurement_key_protocol.RaiseTypeErrorIfNotProvided
The value which has one measurement key.
Determines the fallback behavior when val doesn't have
a measurement key. If default is not set, a TypeError is raised.
If default is set to a value, that value is returned if the value
does not have _measurement_key_name_.
If val has a _measurement_key_name_ method and its result is not
NotImplemented, that result is returned. Otherwise, if a default
value was specified, the default value is returned.
val doesn't have a _measurement_keyname method (or that method
returned NotImplemented) and also no default value was specified.