IsolatePlus is an option for Isolate and
other functions using Isolate. If it is set to
True, Isolate will split sums that contain elements from
vlist, to be able to abbreviate the vlist-free
part.
Isolate[a + b + c + d, a] a+b+c+d
Isolate[a + b + c + d, a, IsolatePlus -> True]a+\text{KK}(19)