subtract(set[2])

Takes a set of precisely 2 series. Uses the first (set[0]) as the minuend and the second (set[1]) as the subtrahend to generate a difference series (i.e. set[0] - set[1]) as the return value.

subtract([
      sum(s("foo",{"host": "prod.web*"})),sum(s("bar",{"host": "prod.web*"}))
])