The `$` operator takes a sequence of FORMS and returns a unary
function which applies the input through that sequence via the `->>`
operator.
For example, consider the predicate "not null". `null` is built into
Common Lisp but "not null" requires writing a
function (lambda (x) (not (null x))). Now, using this operator, you
can write ($ not null) which returns the same lambda as above while
being more concise.
`fn' is a convenience macro for defining functions with a type
specifier. Only really matters for `sbcl` and other hard-optimising
Lisp interpreters which actually take these seriously.
