code: public void addVariable(String attribute, String varName, String value) {
  	Bindings binds = rslTree.getBindings(attribute);
  	if (binds == null) {
  	    binds = new Bindings(attribute);
  	    rslTree.put(binds);
  	}
  	binds.add(new Binding(varName, value));
      }
docs: Adds a new variable definition to the specified variable definitions
  attribute.
  
  @param attribute the variable definitions attribute - rsl_subsititution.
  @param varName the variable name to add.
  @param value the value of the variable to add.