Variable.java~

public class Variable extends Factor implements Comparable{
  
  private String subscript; 
  private char letter; 
   
  
  public Variable(char c){
    this.letter = c; 
    valueMask = new StringBuilder(""+c); 
  }
  
  public char letter(){
   return letter;  
  }
  
  
  public void append(char c){
    throw new IllegalArgumentException("Can't append a variable"); 
  }
  
  public void setSubscript(String s){
    subscript = s; 
  }
  
  
  public void setValue(Constant constant){
    //value = constant; 
  }
  
  public String subscript(){
   return subscript;  
  }
  public int compareTo(Object o){
   
    if(!(o instanceof Variable)){
       throw new IllegalArgumentException("Variables only comparable to other factors.");
    }
       
    
    Variable that = (Variable)o;
    
    if(this.letter < that.letter())
      return -1; 
    else if(this.letter > that.letter())
      return 1; 
    else  
      return operations.compareTo(that.operations);  

    
  }
  
  public Variable(Variable v){
  
    this.letter = v.letter(); 
    operations = new OperationList(v.operations()); 
  }
  
  
  
  
  public String toString(){
   return "("+valueMask + (operations != null ? operations.toString() : "")+")";   
  }
  
}