[racket] Implementing delimited continuations using a CPS transform

It is often used to represent a function that applies on a term, or an
expression.

  [ t ]  means the function [] applied to t.

In those papers, it often represent a CPS transform.

Like [ x ] = \ k . k x would mean "the cps transform of 'x' is '\ k .
k x' , or cps(x) = \ k . k x.

Using [ _ ] is a bit nicer.