ADDED: format/2: support ~:D

The colon modifier for ``D`` forces the use of Prolog digit grouping
using ``_``.

man/builtin.doc

@@ -9758,7 +9758,10 @@ according to the locale of the output stream.  See \secref{locale}.
     \fmtchar{D}
 Same as {\bf d}, but makes large values easier to read by inserting a
 comma every three digits left or right of the dot.  This is the same
-as \verb$~:d$, but using the fixed English locale.
+as \verb$~:d$, but using the fixed English locale.  If \textbf{D} is
+modified using the colon (\verb$~:D$), it uses the Prolog grouping
+character \chr{_}.  See also setup_prolog_integer_grouping/0.  Future
+versions may also support line breaks in big integers.
 
     \fmtchar{e}
 Output next argument as a floating point number in exponential

src/os/pl-fmt.c

@@ -704,9 +704,15 @@ do_format(IOSTREAM *fd, PL_chars_t *fmt, int argc, term_t argv, Module m)
 		    static char grouping[] = {3,0};
 
 		    if ( c == 'D' )
-		    { ltmp.thousands_sep = L",";
-		      ltmp.decimal_point = L".";
-		      ltmp.grouping = grouping;
+		    { if ( mod_colon )
+		      { ltmp.thousands_sep = L"_";
+			ltmp.decimal_point = L".";
+			ltmp.grouping = grouping;
+		      } else
+		      { ltmp.thousands_sep = L",";
+			ltmp.decimal_point = L".";
+			ltmp.grouping = grouping;
+		      }
 		      l = &ltmp;
 		    } else if ( mod_colon )
 		    { l = fd->locale;