CAS.diff command results diff command again

[josephec]

Hi Friends,
I am working with diff command BUT using variable name that stores a function F(X) some results include diff command again

Using diff in HOME direct goes fine:
diff(SEC(X)-1,X)
returns
SIN(X)/COS(X)^2

The problem is if the function is stored in a variable in this case fx
fx:='SEC(X)-1'
diff(fx,X)
returns
diff(SEC(X),X)

returns diff as part of result.... then HOW TO GET the final derivative using variable ?

Pd. I use diff in a program to calculate derivative then show it to user and evaluate in some X points.

Thanks for your answer. Best regards!!
joseph