Type applications are written by calling APPLYTYPE(typ|"C", ...) on a symbol or a tree:
scala> import treehugger.forest._, definitions._, treehuggerDSL._
import treehugger.forest._
import definitions._
import treehuggerDSL._
scala> val tree = REF("put") APPLYTYPE(IntClass) APPLY(LIT(0))
[1m[34mtree[0m: [1m[32mtreehugger.forest.Apply[0m = Apply(TypeApply(Ident(put),List(TypeTree())),List(Literal(Constant(0))))
scala> treeToString(tree)
[1m[34mres0[0m: [1m[32mString[0m = put[Int](0)