There is this method for a MathObject Formula:

`$f->usesOneOf("x",...)`

that tells you whether or not a Formula uses some variable. Is there something similar for functions? My objective is to take a student formula (in a custom answer checker) and determine if they are using 'sqrt'. I don't want to disable 'sqrt', I just want to tailor the custom feedback message to whether or not they used it.

I could just print the string for the student formula and see if 'sqrt' appears in that string. It would feel better if there was something that actually looked in the tree for $f.