In recent work Jim Haglund, Jennifer Morse and Mike Zabrocki introduce a new statistic on Parking Functions, the ``diagonal composition,'' which gives the lengths of the intervals between successive diagonal hits of the Dyck path. They conjectured that the nabla operator, when applied to certain modified Hall-Littlewood functions indexed by compositions, yields the weighted sum of the corresponding Parking Functions by area, dinv, and Gessel quasisymmetric function. This conjecture then gives a sharpening of the ``shuffle conjecture'' and suggests several combinatorial conjectures about the parking functions. In particular, we discuss a bijective map on the parking functions implied by the commutativity properties of the modified Hall-Littlewood polynomials that appear in their conjecture.