module
  di TreeICG(ElementICG e) { 
    Tree : Leaf | Branch .
    Leaf = Data @ e.
    Branch = Data @ e ListOfTree.
    ListOfTree : Empty | NonEmpty.
    Empty = .
    NonEmpty = <f> Tree <r> ListOfTree.
  } 
  aspect TreeWalker { 
    declare strategy
    declare traversal
    declare constraints
  }
  // visitor classes
end
module
  di ElementICG { 
    Data = Name_ValuePair_List.
    NameValuePair = Name Value.
    Name = .
    Value = .
    Name_ValuePair_List : Empty | NonEmpty.
    Empty = .
    NonEmpty = <f> NameValuePair <r> Name_ValuePair_List.
  }
  aspect ElementWalker { 
    declare constraints
    declare strategy
    declare traversal
  }
  // visitor classes
end
module
  di Decide {
    F = Constraints.
    Constraints : E | N.
    E = .
    N = <f> Constraint <r> Constraints.
    Constraint = RelNumber <v1> Variable <v2> Variable.
    RelNumber = int.
    Variable = .
  }
  aspect Chooser {
    declare strategy
    declare constraints
    declare traversal
  }
end
cd XMLSubset implements TreeICG(ElementICG) {
  XMLDef = A <b> B.
  A : X | Y common <u> U V.
  X = Y.

  map treeMap = for TreeICG { 
   use Tree as XMLDef
   use Leaf as EmptyTag
  }
  map elementMap = for ElementICG {
   use Data as OpenTag
   use Data as OpenCloseTag
   // interesting
  }
}