email@example.com (Fred Curtis) wrote: | | Whilst wandering through the MathML specs recently, I came across the | MathML entity &SuchThat which is equated to Unicode #220D, a backwards | greek epsilon.[...] what areas of mathematics have [this]?
This notation was introduced by Peano in 1898, and is still occasionally employed in logic, calculus, etc; cf. a few threads in MAA math-history-list located by searching the archive  for "backward epsilon".
Hopefully Jeff Miller will add this to the appropriate web page  in his pages on earliest uses of math words and symbols.