IN SML PLEASE

5. foldLstTree - 20% (16 %) A polymorphic tree type with nodes of arbitrary number of children might be represented as follows (note that the leafs store list and interior nodes store list of "listTree"s): datatype 'a listTree = lstLEAF of ('a list) I listNODE of ( 'a listTree list) Write a function foldListTree that takes a function (f), a base value (base), and a listTree (t) and combines the values in the lists of the leaf notes in tree t by applying function f. (The leaves of the tree are scanned from left to right) foldListTree is invoked as: foldListTree f base t where f is the combining function of type ' a-> ' a-> ' a . The type of foldListTree should be