The let? macro has the same behavior as let, except where a binding is followed by one or more of keyword clauses.

A contrived example:

(when-let [a foo]
(let [b bar]
(when (even? b)
(let [c baz]
(when (> b c)
(let [d qux]
(f a b c d)))))))

becomes:

(let? [a foo :else nil
b bar :is even?
c baz :when (> b c)
d qux]
(f a b c d))