Returns a new, transient version of the collection, in constant time. Transients support a parallel set of 'changing' operations, with similar names followed by ! - assoc!, conj! etc. These do the same things as their persistent counterparts except the return values are themselves transient. Note in particular that transients are not designed to be bashed in-place. You must capture and use the return value in the next call. In this way, they support the same code structure as the functional persistent code they replace.