(luhn-valid? "79927398713")          ;=> true
(luhn-check-digit "7992739871")       ;=> "3"
(iso7064-mod11-2-check "000000021825009") ;=> "7"   (ORCID)

Standalone check-digit algorithms, usable directly when you need the primitive
rather than a typed identifier validator (the `stdnum.core` parallel to
python-stdnum's `stdnum.luhn` / `stdnum.verhoeff` / `stdnum.iso7064` modules).

Each algorithm offers a `*-valid?` predicate over a complete string (payload +
trailing check character) and, where meaningful, a `*-check-digit` /
`*-check` calculator that returns the check character for a bare payload.

    (luhn-valid? "79927398713")          ;=> true
    (luhn-check-digit "7992739871")       ;=> "3"
    (iso7064-mod11-2-check "000000021825009") ;=> "7"   (ORCID)

(valid?  :iban "GB82 WEST 1234 5698 7654 32")  ;=> true
(parse   :credit-card "4111111111111111")      ;=> {:valid? true :network :visa}
(format  :iban "GB82WEST12345698765432")       ;=> "GB82 WEST 1234 5698 7654 32"
(detect  "4111111111111111")                   ;=> [:credit-card :luhn]

Unified validation, parsing, and formatting of standard identifier numbers.

One API over several identifier types - credit cards, IBAN/BIC, ISBN, ISSN,
ISIN, US bank routing (ABA), IMEI, and the raw Luhn check - dispatched on a
type keyword. Idiomatic Clojure data in and out; an idiomatic facade over the
maintained Apache Commons Validator and iban4j engines (it does not reinvent
the algorithms).

    (valid?  :iban "GB82 WEST 1234 5698 7654 32")  ;=> true
    (parse   :credit-card "4111111111111111")      ;=> {:valid? true :network :visa}
    (format  :iban "GB82WEST12345698765432")       ;=> "GB82 WEST 1234 5698 7654 32"
    (detect  "4111111111111111")                   ;=> [:credit-card :luhn]

`valid?`/`parse`/`format` throw only on an unknown identifier type (a caller
bug); bad input data never throws - `valid?` returns false, `parse` returns
{:valid? false}, `format` returns nil.

(require '[stdnum.vies :as vies])
(vies/check "DE136695976")
;=> {:valid? true, :country "DE", :vat-number "136695976", :name "...", ...}

Online EU VAT validation against the official VIES service
(https://ec.europa.eu/taxation_customs/vies/). This is the one capability in
the library that performs network I/O - it is kept in its own namespace so
`stdnum.core` stays pure and dependency-light. `check` confirms a VAT number
actually *exists* in the member-state registry (and returns the trader name /
address where the state discloses them), which a checksum cannot do.

    (require '[stdnum.vies :as vies])
    (vies/check "DE136695976")
    ;=> {:valid? true, :country "DE", :vat-number "136695976", :name "...", ...}

Requires JDK 11+ (uses java.net.http). On any network/service failure `check`
returns `{:error <message>}` rather than throwing.