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.