(r5rs.info) Delayed evaluation

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 4.2.5 Delayed evaluation
 ------------------------
 
  -- library syntax: delay <expression>
      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  Control features)
      for a more complete description of `delay'.
 
 

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