overloading variables is nothing new in math/science/engineering. Usual methods include picking a different variable to represent torsion, or appending a subscipt or superscript of some kind to differentiate between the two taus. you could also use a bar (like hbar and h in physics) or an apostrophe.
If we're going to be decorating the tau anyway, let's do it from the outset so I don't have to figure out which tau they chose to decorate in this book/paper/whatever.
Prime means derivative; fʹ is the derivative of f, whereas either of f' or f’ is an abomination. (TeX permits the former abomination by, essentially, translating f' into f^\prime.)
