• (spec form & {:keys [gen]})
Takes a single predicate form, e.g. can be the name of a predicate,
like even?, or a fn literal like #(< % 42). Note that it is not
generally necessary to wrap predicates in spec when using the rest
of the spec macros, only to attach a unique generator
 Can also be passed the result of one of the regex ops -
cat, alt, *, +, ?, in which case it will return a regex-conforming
spec, useful when nesting an independent regex.
 Optionally takes :gen generator-fn, which must be a fn of no args that
returns a test.check generator.
 Returns a spec.