In Tcl releases 8.5.0 through 8.5.7, the public header file `tcl.h`
contains the typedef:

===
typedef unsigned long mp_digit
===

Starting in Tcl 8.5.8, this is changed to:

===
typedef unsigned int mp_digit
===

The purpose of this page is to discuss what incompatibilities
this may introduce for C code that uses the Tcl public interface,
and how to manage them.