[brudgers]

Learning advanced mathematical notation requires correlating the notation with the concept.

   < x 

is no less intuitive than

  box x

in regard to the mathematical concepts. Maybe the latter is slightly easier to google...if we can filter out the use of "box" in all sorts of other contexts including those in computing where "box" is some sort of graphic element. Which is to say that "box" is far more overloaded than a monadic use of "<".

We didn't learn Σ intuitively.