Answer the maximum between the receiver and aNumber. Redefine
in subclasses if necessary to ensure that if either self or
aNumber is a NaN, it is always answered.
min: aNumber
Answer the minimum between the receiver and aNumber. Redefine
in subclasses if necessary to ensure that if either self or
aNumber is a NaN, it is always answered.
withSignOf: aNumber
Answer the receiver, with its sign possibly changed to match
that of aNumber.