Traits 

Traits are defined using TRAITDEF(...):

scala> import treehugger.forest._, definitions._, treehuggerDSL._
import treehugger.forest._
import definitions._
import treehuggerDSL._

scala> val tree = TRAITDEF("Philosophical") := BLOCK(
  PROC("philosophize") := BLOCK(
    LIT(0)
  ) 
)
tree: treehugger.forest.ClassDef = ClassDef(Modifiers(<defaultparam/trait>, , Map()),Modifiers(, , Map()),Philosophical,List(),List(),Template(List(),ValDef(Modifiers(private, , Map()),Ident(_),EmptyTree),List(ProcDef(Modifiers(, , Map()),philosophize,List(),List(),Block(List(),Literal(Constant(0)))))))

scala> val tree2 = (CLASSDEF("Animal"): Tree)
tree2: treehugger.forest.Tree = ClassDef(Modifiers(, , Map()),Modifiers(, , Map()),Animal,List(),List(),Template(List(),ValDef(Modifiers(private, , Map()),Ident(_),EmptyTree),List()))

scala> val tree3 = (TRAITDEF("HasLegs"): Tree)
tree3: treehugger.forest.Tree = ClassDef(Modifiers(<defaultparam/trait>, , Map()),Modifiers(, , Map()),HasLegs,List(),List(),Template(List(),ValDef(Modifiers(private, , Map()),Ident(_),EmptyTree),List()))

scala> val tree4 = (CLASSDEF("Frog")
    withParents("Animal", "HasLegs", "Philosophical") := BLOCK(
  DEF(Any_toString) withFlags(Flags.OVERRIDE) := LIT("green")
))
tree4: treehugger.forest.ClassDef = ClassDef(Modifiers(, , Map()),Modifiers(, , Map()),Frog,List(),List(),Template(List(TypeTree(), TypeTree(), TypeTree()),ValDef(Modifiers(private, , Map()),Ident(_),EmptyTree),List(DefDef(Modifiers(override <method>, , Map()),toString,List(),List(),TypeTree(),Literal(Constant(green))))))

These print as:

scala> treeToString(tree)
res0: String = 
trait Philosophical {
  def philosophize {
    0
  }
}

scala> treeToString(tree2)
res1: String = class Animal

scala> treeToString(tree3)
res2: String = trait HasLegs

scala> treeToString(tree4)
res3: String = 
class Frog extends Animal with HasLegs with Philosophical {
  override def toString = "green"
}