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 string : mzlib:string^)
(unit compile : mzlib:compile^)
(unit math : mzlib:math^)
(unit thread : mzlib:thread^)))
The mzlib:core@ unit implements this signature by linking the [CORE] library units together.