align values in a lisp let form

[Kevin Ryde]

Is there a way to align values in a let when editing, ie. to go from

    (let ((x 1)
          (foo 2)
          (zz 3))

to

    (let ((x   1)
          (foo 2)
          (zz  3))

I made myself a bit of code to do it, but I'd be surprised if nobody
else has done something like this before.  align.el doesn't seem to
suit.