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The < method actually is defined as follows for the Magnitude class:
Magnitude method definition • instance < self implementedBySubclass
Evidently, if you send a < message to a Magnitude instance,
and the < method is not overridden by a lower-level class, then the
< method defined for the Magnitude class sends a
implementedBySubclass to self, which produces the appropriate
error display.
Thus, the definition of < for the Magnitude class reminds you
that every subclass of Magnitude is to be equipped with a <
method. You see the corresponding error display if you attempt to use
< without defining it.