Internal functions of libraries can be called as if the library were a
base contract of the calling contract. As the calling convention for
internal functions is to not create a new call context, the code of
these functions will be pulled into the context of the caller,
duplicating their code. This might pull in code of further internal or
even private functions.
The use case for such functions is to allow libraries which can operate
on memory types such that these types can also be modified in place.
