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.