left brackets insert both

[Shug Boabby]

hi there,

a friend of mine was demonstrating a (pay for) text editor he has for
Mac OS X, which had a very nice feature, i am sure it is implementable
in Emacs, but my lisp skills are not good enough to code it. could
somebody please help me implement it?

when the start of a brace pair was typed, one of [<[{(], the editor
would insert the corresponding closing brace and place the cursor
in-between. this worked for all text editing modes, but could be turned
off when needed.

anyone have a clue?