diff -u -r tworld-1.3.0/solution.c tworld-1.3.0-ruleswitch/solution.c
--- tworld-1.3.0/solution.c	2006-04-16 00:49:27.000000000 +0100
+++ tworld-1.3.0-ruleswitch/solution.c	2009-04-06 19:34:00.000000000 +0100
@@ -544,7 +544,7 @@
 
 /* Locate the solution file for the given data file and open it.
  */
-static int opensolutionfile(fileinfo *file, char const *datname, int writable)
+static int opensolutionfile(fileinfo *file, char const *datname, int ruleset, int writable)
 {
     static int	savedirchecked = FALSE;
     char       *buf = NULL;
@@ -562,9 +562,15 @@
 				  && tolower(datname[n - 2]) == 'a'
 				  && tolower(datname[n - 1]) == 't')
 	    n -= 4;
-	xalloc(buf, n + 5);
+	xalloc(buf, n + 7);
 	memcpy(buf, datname, n);
-	memcpy(buf + n, ".tws", 5);
+	/* Use a different filename depending on the ruleset. */
+	if(ruleset == Ruleset_MS)
+		memcpy(buf + n, "-m.tws", 7);
+	else if(ruleset == Ruleset_Lynx)
+		memcpy(buf + n, "-l.tws", 7);
+	else /* Should never happen. */
+		memcpy(buf + n, ".tws\0\0", 7);
 	filename = buf;
     }
 
@@ -597,7 +603,7 @@
 	series->savefile.name = series->savefilename;
     if ((!series->savefile.name && (series->gsflags & GSF_NODEFAULTSAVE))
 		|| !opensolutionfile(&series->savefile,
-				     series->filebase, FALSE)) {
+				     series->filebase, series->ruleset, FALSE)) {
 	series->solheaderflags = 0;
 	series->solheadersize = 0;
 	return TRUE;
@@ -659,7 +665,7 @@
 	series->savefile.name = series->savefilename;
     if (!series->savefile.name && (series->gsflags & GSF_NODEFAULTSAVE))
 	return TRUE;
-    if (!opensolutionfile(&series->savefile, series->filebase, TRUE))
+    if (!opensolutionfile(&series->savefile, series->filebase, series->ruleset, TRUE))
 	return FALSE;
 
     if (!writesolutionheader(&series->savefile, series->ruleset,
diff -u -r tworld-1.3.0/tworld.c tworld-1.3.0-ruleswitch/tworld.c
--- tworld-1.3.0/tworld.c	2006-04-17 09:30:17.000000000 +0100
+++ tworld-1.3.0-ruleswitch/tworld.c	2009-04-06 19:39:54.000000000 +0100
@@ -347,6 +347,8 @@
       case CmdProceed:		*move = CmdProceed;		return FALSE;
       case CmdQuitLevel:	*move = CmdQuitLevel;		return FALSE;
       case CmdHelp:		*move = CmdHelp;		return FALSE;
+       case CmdEast:		*move = CmdEast;		return FALSE;
+       case CmdWest:		*move = CmdWest;		return FALSE;
       case CmdQuit:						exit(0);
 
     }
@@ -1303,6 +1305,16 @@
 		getseriesfromlist(&gs->series, series->list, n);
 		okay = TRUE;
 		break;
+	    } else if (f == CmdEast) {
+		getseriesfromlist(&gs->series, series->list, n);
+		gs->series.ruleset = Ruleset_Lynx;
+		okay = TRUE;
+		break;
+	    } else if (f == CmdWest) {
+		getseriesfromlist(&gs->series, series->list, n);
+		gs->series.ruleset = Ruleset_MS;
+		okay = TRUE;
+		break;
 	    } else if (f == CmdQuitLevel) {
 		okay = FALSE;
 		break;