I try to repeat lines (25)-(28) at

http://www.maplesoft.com/support/help/maple/view.aspx?path=Physics%2fTrace#commentform

I use Maple 14. However, instead of (28) I get the following result:

It means that Maple 14 does not perceive p_\mu, k_\nu and m as scalar quantities. I would like to ask how to define these variables correctly.

Thank you in advance!