Question 5:
===========
4 UNKNOWNs: 10 points each

Consider the slightly modified version of the XPath1 class dictionary
where each expression has a parent has-a edge to the containing expression (see XPath2 below)
in a similar way as each directory in hw 4 had a parent has-a edge to the
containing directory.

The class dictionary violates the DRY principle because ["parent" <parent> Exp]
is repeated three times. What is the easiest way to avoid this duplication?
Put your answer into UNKNOWN1.
Clearly indicate which classes you modify.

class dictionary XPath2:

// Neven-xpath.pdf
import edu.neu.ccs.demeter.dj.*;
import java.util.*;

XPath = List(Exp) EOF.
Exp : Simple | BinaryCompound | UnaryCompound.
Simple : ElementTest | WildCard common ["parent" <parent> Exp].
BinaryCompound = "(" BOp <arg1> Exp <arg2> Exp ")" 
  ["parent" <parent> Exp].
UnaryCompound = "{-{" UOp <arg1> Exp "}-}" 
  ["parent" <parent> Exp].
BOp : Disjunction | Child | Descendant | Filter.
UOp : Chi | Des.
Chi = "/".
Des = "desc".
Disjunction = "|".
Child = "/". 
Descendant = "desc" .
Filter = "filt".
ElementTest = Name.
WildCard = "*".
Name = Ident.

Main = <s> String.
List(S) ~ "(" {S} ")".

CountingVisitor = <c> int  extends Visitor .
ParentSettingVisitor = extends Visitor .
//////////////////////////////////////////////////////////////////

Now that the class dictionary is improved,
the next task is to and write a program that sets the parent links
in a given list of expressions.
The program will visit all Binary- and Unary-objects and 
set the parent links of the contained subexpressions.
Remember that a strategy may have a set of target classes, such as
"from A via B to {C,D,E}.
Find the UNKNOWNs below.


Main {
  public static void main(String args[]) throws Exception {{
    XPath m = XPath.parse(System.in);
    // m.display();
    ClassGraph cgfull = new ClassGraph(true, false);
    ClassGraph cg = new ClassGraph(cgfull,
      "from XPath bypassing -> *,tail,* to *");
    System.out.println("done");

    String parents = "from XPath UNKNOWN2";
    cg.traverse(m,parents,new ParentSettingVisitor());
    m.display();
  }}
}
ParentSettingVisitor {
  {{
  public void before (BinaryCompound host) {
    UNKNOWN3
  }
  public void before (UnaryCompound host) {
    UNKNOWN4
  }
  }}
}

XPath {
 void display() bypassing -> *,parent,* to * (DisplayVisitor); 
}