-- in a movie script
on assert (ok, message)
  if not ok then
    if not voidP(message) then _player.alert(message)
    abort -- exits from current call stack, i.e. also from the caller function
  end if
end

-- anywhere in the code
on test
  x = 42
  assert(x=42, "Assertion 'x=42' failed")
  put "this shows up"
  x = 23
  assert(x=42, "Assertion 'x=42' failed")
  put "this will never show up"
end