The delay construct is used together with the procedure force to
implement lazy evaluation or call by need.
(delay <expression>) returns an object called a
promise which at some point in the future may be asked (by
the force procedure)
to evaluate
<expression>, and deliver the resulting value.
The effect of <expression> returning multiple values
is unspecified.
See the description of force (section see Control features) for a
more complete description of delay .
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