Allow for indefinite products Product(f(x), x)
Implement pretty printing of products with less than 3 indices
Add proper printing of Product objects
Add proper representation of Product objects
Tests for printing of products
New tests for str printing of Sum and Product
Pretty print tests of Sum objects with less than 3 indices
Pretty print tests of Product objects with less than 3 indices
Latex output of Sums and Products with less than 3 indices

This is a proposal and denests multi-limit sums and products.
The reason is that I think the stacked notation will become
rather unreadable with mixed type limits.
Test cases for new latex printing of Sum and Product
Interpret two-index form as Sums/Products over symbolic sets
Test this set interpretation
Interpret two-index form as Sums/Products over symbolic sets
Test this set interpretation