How Can I Evaluate an Expression in Velocity?

[bluejoe]

Hi All! 
 
  In my project, I want to use Velocity to evaluate user-defined expressions 
like:
 
    $a + $b
 
  How can I do that? It seems that I cannot use such an expression in a 
template directly. Then I tried to use #set directive:
 
    #set ($temp = $a + $b)
    $temp
 
  It works well, but I doubt if there is a method provided in Velocity such as:
 
    Object o = Velocity.eval(vc, "$a + $b");
 
  Could you give me an answer on this point? I need your help. Thanks a lot! 
 
Best Regards,
bluejoe