PTC mathcad function

You mean this:

Well, neither t_sub_a nor t_sub_b are defined above or to the left of the definition of this function.

Where did you (think you) defined them?

Success!
Luc

You try to symbolically evaluate a function that is traceable to a function that includes a branch (Phi and F).

You can't symbolically evaluate/display a function which includes a branch (if-function or if-statement).