Help with Cast Sequence and Fel Devastation

What is the correct syntax to activate Fel Devastation, say with alt, but if there isnt enough energy it will fracture instead then activate Fel Devastation?

/castsequence [nochanneling,combat,mod:Shift] Fel Devastation; [nochanneling,combat,nomod] Fracture; [nochanneling,combat,nomod] Fracture