Sharlatan Hellseher <sharlatanus@gmail.com> skribis:

> +;; FIXME: Failed to build on ECL: "fORMFEED" is an illegal character name.An
> +;; error occurred during initialization:
> +;;
> +;; (define-public ecl-boost-json
> +;;  (sbcl-package->ecl-package sbcl-boost-json))

Apparently '#\formfeed' in "decode.lisp" should be replaced by '#\page'.

Even in SBCL:

--8<---------------cut here---------------start------------->8---
* #\formfeed
#\Page
--8<---------------cut here---------------end--------------->8---

You could also report this upstream.