the goal is write a for loop to iterate and get the power of a variable with degree function
i use nops and convert(F, list) can access each terms and variables
however y^2 can be convert to list [y,2]
hope to add if condition to stop convert this.
how to do better?
i just guess to convert it first and then check if there is one of them is IsDigit in StringTools
however IsDigit(2) got error
is there a function to check number of variable in it?