Add △ as synonym for `symdiff`
Add negated element containments ∉, ∌
More comparsion operators ≠ (!=), ≡ (===), ≢ (!==), ≤ (<=), ≥ (>=), .≥ (.>=), .≤ (<=), .≠ (.!=)
Add double solidus (⫽) as synonym for rational division (//)

Is (.//) defined anywhere?
Adds ∣ (divides) and its negation ∤
Add × (cross product) as synonym for `cross`