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"unspecified" and SET!

    Date: 18 Jun 89 23:17:50 GMT
    From: Dorai Sitaram <titan!dorai@rice.edu>
    Perhaps, SET! is not the ideal side-effecting construct for an
    expression-oriented language. Felleisen and Friedman [1] propose a
    side-effecting construct called a SIGMA-capability. This looks just
    like a LAMBDA-expression, but whereas LAMBDA introduces new bound
    variables, SIGMA modifies the binding of existing lexical variables.
    In other words, on applying a SIGMA-capability to arguments, the
    variables corresponding to the SIGMA-parameters are side-effected.

    Problems about "unspecified" return values disappear....

I'm having a hard time applying this idea to the NEWLINE procedure -- which
is specified in R3RS to return an unspecified value.  Is the idea to
introduce a side-effecting construct that looks like LAMBDA, but performs a
newline when applied to an output port?

  ((NULINE () <body>)