The mzlib:core^ signature is defined by:
(define-signature mzlib:core^ ((unit pretty-print@ : mzlib:pretty-print^) (unit file@ : mzlib:file^) (unit function@ : mzlib:function^) (unit compat@ : mzlib:compat^) (unit string@ : mzlib:string^) (unit compile@ : mzlib:compile^) (unit thread@ : mzlib:thread^)))
The mzlib:core@ unit implements this signature by linking the [CORE] library units together.