Class RealStepper

Instance Methods

Return a new stepper which steps independently of me, but whose current
value is the same as mine, and which must produce a future history of values
which satisfies the same obligation that my contract obligates me to produce
now. Typically, this will mean that he must produce the same future history
that I'm going to produce. However, let's say that I am enumerating the
elements of a partial order in some full order which is consistent with the
partial order. If a copy of me is made after I'm part way through, then me
and my copy may produce any future history compatable both with the partial
order and the elements I've already produced by the time of the copy. Of
course, a subclass or a Stepper creating message (like
IntegerRegion::stepper()) may specify the more stringent requirement (that a
copy must produce the same sequence).

To prevent aliasing, Steppers should typically be passed by copy. See class
comment.

Overrides: Stepper

If I am exhausted (ie., if (! this->hasValue())), then return nil. Else return
current element. I return wimpily since most items returned are held by collections.
If I create a new object, I should cache it.

Overrides: Stepper

Iff I have a current value (ie. this message returns true), then I am not
exhasted. 'fetch' and 'get' will both return this value, and I can be 'step'ped to
my next state. As I am stepped, eventually I may become exhausted (the
reverse of all the above), which is a permanent condition.

Note that not all steppers have to be exhaustable. A Stepper which
enumerates all primes is perfectly reasonable. Assuming otherwise will create
infinite loops. See class comment.

Overrides: Stepper

Essential. If I am currently exhausted (see Stepper::hasValue()), then it is an error to step me. The result of doing so isn't currently specified (we probably should specify it to BLAST, but I know that the implementation doesn't currently live up to that spec).

If I am not exhausted, then this advances me to my next state. If my current value (see Stepper::get()) was my final value, then I am now exhausted, otherwise my new current value is the next value.

Overrides: Stepper