title: CS-SWAP
description:

CS-SWAP             "c-s-swap"

  Interpretation: Interpretation semantics for this word are undefined.

  Execution: ( C: orig1|dest1 orig2|dest2 -- orig2|dest2 orig1|dest1 )

  Swap two elements on top of the control-flow stack. An ambiguous condition exists if there are less than two items, each of which shall be an orig or dest, at the top of the control-flow stack before CS-SWAP is executed.

discussion:

This word was propposed for ANS standardization, but was rejected because it may be added to any standard program with a single line of code:

: CS-SWAP
    1 CS-ROLL
;

On the other hand, the need to invoke CS-ROLL with an argument other than 1 happens very rarely. You do not really need to implement CS-ROLL to implement ELSE :

: ELSE
    POSTPONE AHEAD CS-SWAP POSTPONE THEN
; IMMEDIATE

because the implementation-dependent definition of CS-SWAP most likely will be:

: CS-SWAP
    2SWAP    \ assuming double-cell orig's and dest's
             \ assuming that the control-flow stack is the data stack
;