Fri Apr 18 17:16:26 EDT 2008

composable continuations


  ... four classes of delimited continuation operators ...  are
  referred to as -F-, -F+, +F- and +F+. Dybvig et al. describes them
  as "a classification of control operators in terms of four variants
  of F that differ according to whether the continuation-capture
  operator (a) leaves behind the prompt on the stack after capturing
  the continuation and (b) includes the prompt at the base of the
  captured subcontinuation."

that makes things a lot easier to understand.