Takes a function and optional arguments, and returns a derefable containing the output of that function. If the function derefs Reagent atoms (or track, etc), the value will be updated whenever the atom changes. In other words, @(track foo bar) will produce the same result as (foo bar), but foo will only be called again when the atoms it depends on changes, and will only trigger updates of components when its result changes. track is lazy, i.e the function is only evaluated on deref.