*Script bang !amptoggle
*Script exec !IfEval ("%{amp}" = "1") '!ampoff' else '!ampon'
*Script exec !IfEval ("%{amp}" = "1") '!varset amp 0' else '!varset amp 1'
*Script exec !varsave amp
*Script ~bang

note: !varsave amp may not be necessary if you're autosaving. The !varset stuff could, of course, be moved to the appropriate !ampon/!ampoff script entries, if you wanted.

If you can't get it from here... I can't explain it any better. =)

(mzscript-0.9.13 - syntax may vary)