check that a rune exists and is enabled
Major Section:  THEORIES

(active-runep '(:rewrite left-to-right))

General Form:
(active-runep rune)
where rune has the shape of a rune. This macro expands to an expression using the variables ens and state, and returns non-nil when the given rune exists and is enabled (according to the given ``enabled structure,'' ens, and the current logical world of the given state). See theory-invariant for how this macro can be of use.