stack overflow => segfault in MT

[<don...@is...>]

I send this on the theory that any segfault qualifies as a clisp bug.
This is with current source.  On non-MT I get 
  *** - Program stack overflow. RESET 
whereas in MT I get segfault.
This is, of course, caused by a bug in my program.
In this case I was lucky and got a warning at each recursion,
so was able to break by setting CUSTOM:*BREAK-ON-WARNINGS* to T.
Here's the backtrace a few recursions into the process that 
eventually ends in segfault.

 > Break 1 AP5[8]> :bt
 > <1/3091> #<SYSTEM-FUNCTION EXT:SHOW-STACK> 3
 > <2/3084> #<COMPILED-FUNCTION SYSTEM::PRINT-BACKTRACE>
 > <3/3078> #<COMPILED-FUNCTION SYSTEM::DEBUG-BACKTRACE>
 > <4/3069> #<SYSTEM-FUNCTION SYSTEM::READ-EVAL-PRINT> 2
 > <5/3066> #<COMPILED-FUNCTION SYSTEM::BREAK-LOOP-2-3>
 > <6/3062> #<SYSTEM-FUNCTION SYSTEM::SAME-ENV-AS> 2
 > <7/3048> #<COMPILED-FUNCTION SYSTEM::BREAK-LOOP-2>
 > <8/3046> #<SYSTEM-FUNCTION SYSTEM::DRIVER>
 > <9/3006> #<COMPILED-FUNCTION SYSTEM::BREAK-LOOP>
 > <10/2989> #<COMPILED-FUNCTION SYSTEM::WARN-OF-TYPE>
 > <11/2981> #<COMPILED-FUNCTION SYSTEM::C-WARNING>
 > <12/2976> #<COMPILED-FUNCTION SYSTEM::C-WARN>
 > <13/2973> #<COMPILED-FUNCTION SYSTEM::NOTE-FUNCTION-USED>
 > <14/2969> #<COMPILED-FUNCTION SYSTEM::C-GLOBAL-FUNCTION-CALL>
 > <15/2948> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <16/2940> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <17/2919> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <18/2838> #<COMPILED-FUNCTION SYSTEM::C-FUNCALL-INLINE>
 > <19/2832> #<COMPILED-FUNCTION SYSTEM::C-FUNCTION-CALL>
 > <20/2829> #<COMPILED-FUNCTION SYSTEM::C-FUNCALL>
 > <21/2808> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <22/2795> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <23/2790> #<COMPILED-FUNCTION SYSTEM::COLLECT-ARGS>
 > <24/2772> #<COMPILED-FUNCTION SYSTEM::C-GLOBAL-FUNCTION-CALL>
 > <25/2751> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <26/2746> #<COMPILED-FUNCTION SYSTEM::COLLECT-ARGS>
 > <27/2728> #<COMPILED-FUNCTION SYSTEM::C-GLOBAL-FUNCTION-CALL>
 > <28/2707> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <29/2703> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <30/2682> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <31/2640> #<COMPILED-FUNCTION SYSTEM::C-LET/LET*>
 > <32/2638> 
 > #<COMPILED-FUNCTION
 >   #:|2105 2263 (DEFCONSTANT C-FORM-TABLE (LET # # ...))-172-1|>
 > <33/2617> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <34/2613> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <35/2592> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <36/2505> #<COMPILED-FUNCTION SYSTEM::C-LAMBDABODY>
 > <37/2493> #<COMPILED-FUNCTION SYSTEM::C-FUNCTION>
 > <38/2472> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <39/2467> #<COMPILED-FUNCTION SYSTEM::COLLECT-ARGS>
 > <40/2459> #<COMPILED-FUNCTION SYSTEM::C-NORMAL-FUNCTION-CALL>
 > <41/2445> #<COMPILED-FUNCTION SYSTEM::C-GLOBAL-FUNCTION-CALL>
 > <42/2424> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <43/2402> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <44/2398> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <45/2377> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <46/2290> #<COMPILED-FUNCTION SYSTEM::C-LAMBDABODY>
 > <47/2281> #<COMPILED-FUNCTION SYSTEM::COMPILE-LAMBDABODY>
 > <48/2201> #<COMPILED-FUNCTION COMMON-LISP:COMPILE>
 > <49/2194> #<COMPILED-FUNCTION COMPILE>
 > <50/2188> #<COMPILED-FUNCTION MYCOMPILE>
 > <51/2185> #<COMPILED-FUNCTION COMPILE-AP>
 > <52/2181> #<COMPILED-FUNCTION NEED-TESTER>
 > <53/2170> #<COMPILED-FUNCTION TRANSLATE??>
 > <54/2165> #<COMPILED-FUNCTION ??>
 > <55/2165> #<SYSTEM-FUNCTION FUNCALL> 3
 > <56/2163> #<SYSTEM-FUNCTION MACROEXPAND>
 > <57/2150> #<COMPILED-FUNCTION MAKE-TEST-GENERATOR>
 > <58/2141> #<COMPILED-FUNCTION FINDGENERATORS>
 > <59/2132> #<COMPILED-FUNCTION FINDGENERATOR>
 > <60/2111> #<COMPILED-FUNCTION GET-GENERATOR>
 > <61/2103> #<COMPILED-FUNCTION ANALYZE-DESC>
 > <62/2097> #<COMPILED-FUNCTION GENERATOR>
 > <63/2097> #<SYSTEM-FUNCTION FUNCALL> 3
 > <64/2095> #<SYSTEM-FUNCTION MACROEXPAND>
 > <65/2080> #<COMPILED-FUNCTION FORANY>
 > <66/2080> #<SYSTEM-FUNCTION FUNCALL> 3
 > <67/2053> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <68/2045> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <69/2024> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <70/1943> #<COMPILED-FUNCTION SYSTEM::C-FUNCALL-INLINE>
 > <71/1937> #<COMPILED-FUNCTION SYSTEM::C-FUNCTION-CALL>
 > <72/1934> #<COMPILED-FUNCTION SYSTEM::C-FUNCALL>
 > <73/1913> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <74/1900> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <75/1895> #<COMPILED-FUNCTION SYSTEM::COLLECT-ARGS>
 > <76/1877> #<COMPILED-FUNCTION SYSTEM::C-GLOBAL-FUNCTION-CALL>
 > <77/1856> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <78/1851> #<COMPILED-FUNCTION SYSTEM::COLLECT-ARGS>
 > <79/1833> #<COMPILED-FUNCTION SYSTEM::C-GLOBAL-FUNCTION-CALL>
 > <80/1812> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <81/1808> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <82/1787> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <83/1745> #<COMPILED-FUNCTION SYSTEM::C-LET/LET*>
 > <84/1743> 
 > #<COMPILED-FUNCTION
 >   #:|2105 2263 (DEFCONSTANT C-FORM-TABLE (LET # # ...))-172-1|>
 > <85/1722> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <86/1718> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <87/1697> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <88/1610> #<COMPILED-FUNCTION SYSTEM::C-LAMBDABODY>
 > <89/1598> #<COMPILED-FUNCTION SYSTEM::C-FUNCTION>
 > <90/1577> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <91/1572> #<COMPILED-FUNCTION SYSTEM::COLLECT-ARGS>
 > <92/1564> #<COMPILED-FUNCTION SYSTEM::C-NORMAL-FUNCTION-CALL>
 > <93/1550> #<COMPILED-FUNCTION SYSTEM::C-GLOBAL-FUNCTION-CALL>
 > <94/1529> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <95/1507> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <96/1503> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <97/1482> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <98/1395> #<COMPILED-FUNCTION SYSTEM::C-LAMBDABODY>
 > <99/1386> #<COMPILED-FUNCTION SYSTEM::COMPILE-LAMBDABODY>
 > <100/1306> #<COMPILED-FUNCTION COMMON-LISP:COMPILE>
 > <101/1299> #<COMPILED-FUNCTION COMPILE>
 > <102/1293> #<COMPILED-FUNCTION MYCOMPILE>
 > <103/1290> #<COMPILED-FUNCTION COMPILE-AP>
 > <104/1286> #<COMPILED-FUNCTION NEED-TESTER>
 > <105/1275> #<COMPILED-FUNCTION TRANSLATE??>
 > <106/1270> #<COMPILED-FUNCTION ??>
 > <107/1270> #<SYSTEM-FUNCTION FUNCALL> 3
 > <108/1268> #<SYSTEM-FUNCTION MACROEXPAND>
 > <109/1255> #<COMPILED-FUNCTION MAKE-TEST-GENERATOR>
 > <110/1246> #<COMPILED-FUNCTION FINDGENERATORS>
 > <111/1237> #<COMPILED-FUNCTION FINDGENERATOR>
 > <112/1216> #<COMPILED-FUNCTION GET-GENERATOR>
 > <113/1208> #<COMPILED-FUNCTION ANALYZE-DESC>
 > <114/1202> #<COMPILED-FUNCTION GENERATOR>
 > <115/1202> #<SYSTEM-FUNCTION FUNCALL> 3
 > <116/1200> #<SYSTEM-FUNCTION MACROEXPAND>
 > <117/1185> #<COMPILED-FUNCTION FORANY>
 > <118/1185> #<SYSTEM-FUNCTION FUNCALL> 3
 > <119/1158> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <120/1150> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <121/1129> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <122/1048> #<COMPILED-FUNCTION SYSTEM::C-FUNCALL-INLINE>
 > <123/1042> #<COMPILED-FUNCTION SYSTEM::C-FUNCTION-CALL>
 > <124/1039> #<COMPILED-FUNCTION SYSTEM::C-FUNCALL>
 > <125/1018> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <126/1005> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <127/1000> #<COMPILED-FUNCTION SYSTEM::COLLECT-ARGS>
 > <128/982> #<COMPILED-FUNCTION SYSTEM::C-GLOBAL-FUNCTION-CALL>
 > <129/961> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <130/956> #<COMPILED-FUNCTION SYSTEM::COLLECT-ARGS>
 > <131/938> #<COMPILED-FUNCTION SYSTEM::C-GLOBAL-FUNCTION-CALL>
 > <132/917> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <133/913> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <134/892> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <135/850> #<COMPILED-FUNCTION SYSTEM::C-LET/LET*>
 > <136/848> 
 > #<COMPILED-FUNCTION
 >   #:|2105 2263 (DEFCONSTANT C-FORM-TABLE (LET # # ...))-172-1|>
 > <137/827> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <138/823> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <139/802> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <140/715> #<COMPILED-FUNCTION SYSTEM::C-LAMBDABODY>
 > <141/703> #<COMPILED-FUNCTION SYSTEM::C-FUNCTION>
 > <142/682> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <143/677> #<COMPILED-FUNCTION SYSTEM::COLLECT-ARGS>
 > <144/669> #<COMPILED-FUNCTION SYSTEM::C-NORMAL-FUNCTION-CALL>
 > <145/655> #<COMPILED-FUNCTION SYSTEM::C-GLOBAL-FUNCTION-CALL>
 > <146/634> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <147/612> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <148/608> #<COMPILED-FUNCTION SYSTEM::C-PROGN>
 > <149/587> #<COMPILED-FUNCTION SYSTEM::C-FORM>
 > <150/500> #<COMPILED-FUNCTION SYSTEM::C-LAMBDABODY>
 > <151/491> #<COMPILED-FUNCTION SYSTEM::COMPILE-LAMBDABODY>
 > <152/411> #<COMPILED-FUNCTION COMMON-LISP:COMPILE>
 > <153/404> #<COMPILED-FUNCTION COMPILE>
 > <154/398> #<COMPILED-FUNCTION MYCOMPILE>
 > <155/395> #<COMPILED-FUNCTION COMPILE-AP>
 > <156/391> #<COMPILED-FUNCTION NEED-TESTER>
 > <157/380> #<COMPILED-FUNCTION TRANSLATE??>
 > <158/375> #<COMPILED-FUNCTION ??>

 ...