# key: //
# name: Fraction auto
# type: command
# condition: (and (texmathp) (quote auto))
# group: math
# --
(let ((numerator t))
  (condition-case nil
      (save-excursion
        (backward-sexp)
        (kill-sexp)
        (delete-char 1))
    (error (setq numerator 'nil)))
  (insert "\\frac{" (if numerator (current-kill 0) "")
          "}{}")
  (backward-char (if numerator 1 3)))