I suggest to add this, which I keep adding to my org-user-entities: (("frac23" "\\frac23" t "⅔" "2/3" "⅔" "⅔") ("over" "\\over" t "/" "/" "/" "/") ("iff" "\\iff" t "⇔" "<=>" "<=>" "⇔") ("implies" "\\implies" t "⇒" "=>" "=>" "⇒"))
The two first just allow to type simple fractions more readably, the two last add semantic variants to the already existing Rightarrow and Rightleftarrow.