code: public TupleRef addVarBinding( VarBinding varBinding)
      {
      assert varBinding != null;
      assert varBinding.getVar() != null;
  
      String varName = varBinding.getVar();
      if( varBindings_.containsKey( varName))
        {
        throw new IllegalStateException( "Value for " + varName + " already defined");
        }
      
      varBindings_.put( varName, varBinding);
      return this;
      }
docs: Adds a new variable binding.