The mzlib/unit library mostly re-provides
scheme/unit, except for struct and
struct/ctc from scheme/unit.
| (struct id (field-id ...) omit-decl ...) | 
|   | 
| omit-decl |   | = |   | -type |  |   |   | | |   | -selectors |  |   |   | | |   | -setters |  |   |   | | |   | -constructor |  
  | 
A signature form like 
struct from 
scheme/unit,
but with a different syntax for the options that limit exports.
| (struct/ctc id ([field-id contract-expr] ...) omit-decl ...) | 
|   | 
| omit-decl |   | = |   | -type |  |   |   | | |   | -selectors |  |   |   | | |   | -setters |  |   |   | | |   | -constructor |  
  | 
A signature form like 
struct/ctc from 
scheme/unit,
but with a different syntax for the options that limit exports.
Like 
struct~s and 
struct~s/ctc, but the constructor is
named the same as the type, instead of with  
make- prefix.