code: public SDVariable arg(int num){
          SDVariable[] args = args();
          Preconditions.checkNotNull(args, "Arguments are null for function %s", this.getOwnName());
          Preconditions.checkArgument(num >= 0 && num < args.length, "Invalid index: must be 0 to numArgs (0 <= idx < %s)", args.length);
          return args[num];
      }
docs: Return the specified argument for this function
  @param num Number of the argument. Must be in range 0 to numArgs - 1 inclusive
  @return Specified argument