The following routines in Tcl's public interface, `tcl.h`, make
use of the ''mp_int'' type:

   * '''Tcl_NewBignumObj'''(''mp_int *'')
   * '''Tcl_DbNewBignumObj'''(''mp_int *'', ''const char *'', ''int'')
   * '''Tcl_SetBignumObj'''(''Tcl_Obj *'', ''mp_int *'')
   * '''Tcl_GetBignumFromObj'''(''Tcl_Interp *'', ''Tcl_Obj *'', ''mp_int *'')
   * '''Tcl_TakeBignumFromObj'''(''Tcl_Interp *'', ''Tcl_Obj *'', ''mp_int *'')
   * '''Tcl_InitBignumFromDouble'''(''Tcl_Interp *'', ''double'', ''mp_int *'')

Tcl's public header, `tcl.h` also includes these declarations:

===
#ifndef MP_INT_DECLARED
typedef struct mp_int mp_int;
#define MP_INT_DECLARED
#endif
#ifndef MP_DIGIT_DECLARED
typedef unsigned long mp_digit;
#define MP_DIGIT_DECLARED
#endif
===

which allows the compiler to make sense of the function declarations.

Callers of those functions must be able to allocate an ''mp_int''
struct so they can pass its address in.  In order to do this a
definition of the ''mp_int'' struct has to be in scope, but Tcl's
public header `tcl.h` does not provide one.

Q1:  What header file are callers of these routines expected
to #include to get a suitable ''mp_int'' definition in scope?

Q2: If the answer is something other than a corrected `tcl.h`,
how are we to verify that the ''mp_digit'' definition active
wherever that definition comes from is the same as that in
`tcl.h`?  ('' Failure to make them consistent leads to a binary
incompatibility'').

Several of Tcl's own source code files need to call these routines.
They have the same problem.  Their solution is `#include "tommath.h"`.
When Tcl sources are built, the `-I` compiler options are set so
that this refers to the file `tcl/generic/tommath.h` which is only
a wrapper pulling in `tcl/generic/tclTomMath.h` which is a patched
version of the original header of libtommath, `tcl/libtommath/tommath.h`.
That's the Tcl internals answer to Q1.

Each Tcl source file using the ''mp_int'' struct, follows the pattern
of `#include "tcl.h"` ''before'' `#include "tommath.h"`.  Also the
file `tcl/generic/tclTomMath.h` has been patched so that when
'''MP_DIGIT_DECLARED''' is defined, any declarations of ''mp_digit''
are skipped.  This combination is the Tcl internals answer to Q2.

''more to come...''