A class, generic function, or method may or may not be **explicitly known** to a given library. A sealing restriction may limit the set of classes, generic functions, and methods to those that are explicitly known; others cannot be added.

- A class
*C*is explicitly known in a library*L*if it is defined by`define class`

in*L*or in a library used by*L*. - A generic function
*G*is explicitly known in a library*L*if*G*is defined by`define generic`

in the library or in one of the libraries*L*uses, or if*G*is implicitly defined by the definition of a method explicitly known in*L*or if*G*is implicitly defined by a slot specification for a class explicitly known in*L*. - A method
*M*is explicitly known in a library*L*if*M*is defined by`define method`

in*L*or in one of the libraries*L*uses, or if*M*is defined by a slot specification for a class explicitly known in*L*.