You might also want to go for something related to the function instead of the name. EG:

http://i.imgur.com/anU03Kk.png

Alternatively, translation icons can provide some inspiration:

https://cdn3.iconfinder.com/data/icons/text-editor-1/24/Translate-512.png

On Friday, December 9, 2016 at 5:54:50 AM UTC-5, John MacFarlane wrote:

> In recall that in some Polish notation oriented logical systems, the
> universal quantifier is marked by a Pi (product Π); and one particular
> attic numeral is an uppercase delta inside an uppercase pi, meaning 50
> (pente deka). It's found in slot "02 in the TeX LGR encoding. So you
> may not need to design anything new.

Clever idea, but it looks a bit too much like a hangman's noose!
https://en.wikipedia.org/wiki/Attic_numerals#/media/File:Attic_00050.svg