amend recalculation of size metadata in 'PersistentVector.splitRIght: we always want to decrement by prevSize in this function. (We will have another function call to decrement so that left split and right split represent the same absolute indices when they are joined, before they are split.)

fcore/persistent-vector.sml

@@ -443,7 +443,7 @@ struct
               else
                 let
                   val nodes = VectorSlice.vector nodesSlice
-                  val sizes = VectorSlice.map (fn el => el - prevSize) sizesSlice
+                  val sizes = VectorSlice.map (fn el => el - oldChildSize) sizesSlice
                 in
                   BRANCH (nodes, sizes)
                 end
@@ -487,11 +487,16 @@ struct
                 empty
               else
                 let
+                  val prevSize =
+                    if idx > 0 then
+                      Vector.sub (sizes, idx - 1)
+                    else
+                      0
                   val len = Vector.length items - idx
                   val itemsSlice = VectorSlice.slice (items, idx, SOME len)
                   val items = VectorSlice.map 
                     (fn {start, finish} => 
-                      {start = start - splitIdx, finish = finish - splitIdx}
+                      {start = start - prevSize, finish = finish - prevSize}
                     ) 
                     itemsSlice
                   val sizes = Vector.map #finish items