> Also, a small prize (a "thank you!" from a stranger on the Internet i.e. me) to someone who can propose a good syntax for compound assignment with reversed subtraction:

I would do away with operator-assign operators and instead introduce a general syntax for updating a variable that can be used with any expression.

  x = _ + 1  # increment x
  x = 0 - _  # negate x
  output = sanitizeHtml(_)
  index = (_ + 1) % len(arr)