If you use <f:number> in forms, you are able to input operators like + or -, but only two of them.
e is an allowed input too, I assume it refers to e as constant in this case?
Though, if you input e, the form complains about you not providing a number.

This is replicable on https://weekly.ci.jenkins.io/design-library/Progress/, if you head over to progressive rendering and find factors of a number.