This post misses the IMO best indentation scheme for lisp, which I used for my college class where we had to use MIT scheme:

    (define (match:element variable restrictions)
        (define (ok? datum)
            (every 
                (lambda (restriction)
                    (restriction datum)
                )
                restrictions
            )
        )
        (define (element-match data dictionary succeed)
            (and 
                (ok? data)
                (let ((vcell (match:lookup variable dictionary)))
                    (if vcell
                        (and 
                            (equal? (match:value vcell) data)
                            (succeed dictionary)
                        )
                        (succeed (match:bind variable data dictionary))
                    )
                )
            )
        )
        element-match
    )

It may not be densest or most compact indentation scheme, but damn is it readable for someone without a lisp/scheme background!