failed AVER: (NOT (EQL # DELETED))
This is probably a bug in SBCL itself. (Alternatively, SBCL
might have been corrupted by bad user code, e.g. by an undefined
Lisp operation like (FMAKUNBOUND 'COMPILE), or by stray pointers
from alien code or from unsafe Lisp code; or there might be a
bug in the OS or hardware that SBCL is running on.) If it seems
to be a bug in SBCL itself, the maintainers would like to know
about it. Bug reports are welcome on the SBCL mailing lists,
which you can find at <http://sbcl.sourceforge.net/>.
   [Condition of type SB-INT:BUG]

Restarts:
 0: [TRY-RECOMPILING] Try recompiling folder
 1: [RETRY] Retry performing #<ASDF:COMPILE-OP NIL {B1683E9}> on #<ASDF:CL-SOURCE-FILE "folder" {FA46161}>.
 2: [ACCEPT] Continue, treating #<ASDF:COMPILE-OP NIL {B1683E9}> on #<ASDF:CL-SOURCE-FILE "folder" {FA46161}> as having been successful.
 3: [ABORT] Abort compilation.
 4: [ABORT] Return to SLIME's top level.
 5: [TERMINATE-THREAD] Terminate this thread (#<THREAD "repl-thread" RUNNING {AF9BE59}>)

Backtrace:
  0: (SB-INT:BUG "~@<failed AVER: ~2I~_~A~:>")[:EXTERNAL]
  1: (SB-IMPL::%FAILED-AVER (NOT (EQL (SB-C::FUNCTIONAL-KIND SB-C::FUNCTIONAL) :DELETED)))
  2: (SB-C::IR2-CONVERT-CLOSURE ..)
  3: (SB-C::IR2-CONVERT-REF #<SB-C::REF :%SOURCE-NAME SB-C::THING :LEAF # {AC7CC79}> #<SB-C::IR2-BLOCK >)
  4: (SB-C::IR2-CONVERT-BLOCK #<SB-C::CBLOCK 122 :START c1 {ACAAB51}>)
  5: (SB-C:IR2-CONVERT #<SB-C:COMPONENT :NAME (FLET #:CLEANUP-FUN-[READ-QUOTED-STRING]387) {AC13629}>)
  6: (SB-C::%COMPILE-COMPONENT #<SB-C:COMPONENT :NAME (FLET #:CLEANUP-FUN-[READ-QUOTED-STRING]387) {AC13629}>)
  7: (SB-C::COMPILE-COMPONENT #<SB-C:COMPONENT :NAME (FLET #:CLEANUP-FUN-[READ-QUOTED-STRING]387) {AC13629}>)
  8: (SB-C::%COMPILE ..)[:EXTERNAL]
  9: (SB-C::FOPCOMPILE-FUNCTION ..)
 10: (SB-C::FOPCOMPILE ..)
 11: (SB-C::CONVERT-AND-MAYBE-COMPILE ..)
 12: ((FLET SB-C::DEFAULT-PROCESSOR) ..)
 13: (SB-C::PROCESS-TOPLEVEL-FORM ..)
 14: (SB-C::PROCESS-TOPLEVEL-PROGN ..)
 15: (SB-C::PROCESS-TOPLEVEL-FORM ..)
 16: (SB-C::PROCESS-TOPLEVEL-PROGN ..)
 17: (SB-C::PROCESS-TOPLEVEL-FORM ..)
 18: ((FLET SB-C::DEFAULT-PROCESSOR) ..)
 19: (SB-C::PROCESS-TOPLEVEL-FORM ..)
 20: (SB-C::SUB-SUB-COMPILE-FILE #<SB-C::SOURCE-INFO >)
 21: ((LAMBDA ()))
 22: (SB-C::%WITH-COMPILATION-UNIT #<CLOSURE (LAMBDA #) {B4F3F55}>)[:EXTERNAL]
 23: (SB-C::SUB-COMPILE-FILE #<SB-C::SOURCE-INFO >)
 24: (COMPILE-FILE #P"/home/andy/src/clbuild/source/mel-base/folders/imap/folder.lisp")[:EXTERNAL]
 25: ((SB-PCL::FAST-METHOD ASDF:PERFORM (ASDF:COMPILE-OP ASDF:CL-SOURCE-FILE)) #<unavailable argument> #<unavailable argument> #<ASDF:COMPILE-OP NIL {B1683E9}> #<ASDF:CL-SOURCE-FILE "folder" {FA46161}>)
 26: ((LAMBDA (SB-PCL::.PV. SB-PCL::.NEXT-METHOD-CALL. SB-PCL::.ARG0. SB-PCL::.ARG1.)) #<unused argument> #<unused argument> #<ASDF:COMPILE-OP NIL {B1683E9}> #<ASDF:CL-SOURCE-FILE "folder" {FA46161}>)
 27: ((SB-PCL::FAST-METHOD ASDF:PERFORM ASDF:AROUND (ASDF:COMPILE-OP ASDF:CL-SOURCE-FILE)) ..)
 28: ((LAMBDA ()))
 29: ((FLET SB-THREAD::WITH-RECURSIVE-LOCK-THUNK))
 30: ((FLET #:WITHOUT-INTERRUPTS-BODY-[CALL-WITH-RECURSIVE-LOCK]317))
 31: (SB-THREAD::CALL-WITH-RECURSIVE-LOCK ..)
 32: (SB-C::%WITH-COMPILATION-UNIT #<CLOSURE (LAMBDA #) {B4E3855}>)[:EXTERNAL]
 33: (ASDF:OPERATE ASDF:LOAD-OP "mel-base")[:EXTERNAL]
 34: ((LAMBDA ()))
 35: ((LAMBDA (FUNCTION)) #<CLOSURE (LAMBDA #) {B130E3D}>)
 36: (SWANK-BACKEND:CALL-WITH-COMPILATION-HOOKS #<CLOSURE (LAMBDA #) {B130E3D}>)
 37: ((FLET #:FORM-FUN-[OPERATE-ON-SYSTEM]135))
 38: (SWANK::OPERATE-ON-SYSTEM "mel-base" SWANK-IO-PACKAGE::LOAD-OP)[:EXTERNAL]
 39: ((LAMBDA ()))
 40: ((LAMBDA ()))
 41: (SWANK::MEASURE-TIME-INTERVAL #<CLOSURE (LAMBDA #) {B130DD5}>)
 42: (SWANK::COLLECT-NOTES #<CLOSURE (LAMBDA #) {B130DB5}>)
 43: (SWANK:OPERATE-ON-SYSTEM-FOR-EMACS "mel-base" SWANK-IO-PACKAGE::LOAD-OP)[:EXTERNAL]
 44: (SB-INT:SIMPLE-EVAL-IN-LEXENV (SWANK:OPERATE-ON-SYSTEM-FOR-EMACS "mel-base" 'SWANK-IO-PACKAGE::LOAD-OP) #<NULL-LEXENV>)
 45: (SWANK::EVAL-FOR-EMACS (SWANK:OPERATE-ON-SYSTEM-FOR-EMACS "mel-base" 'SWANK-IO-PACKAGE::LOAD-OP) "MAIL-TIPIZZATE" 119)
 46: (SWANK::PROCESS-REQUESTS NIL)
 47: ((LAMBDA ()))
 48: (SWANK-BACKEND::CALL-WITH-BREAK-HOOK #<FUNCTION SWANK:SWANK-DEBUGGER-HOOK> #<CLOSURE (LAMBDA #) {BD1FD9D}>)
 49: ((LAMBDA (SWANK-BACKEND::HOOK SWANK-BACKEND::FUN)) #<FUNCTION SWANK:SWANK-DEBUGGER-HOOK> #<CLOSURE (LAMBDA #) {BD1FD9D}>)
 50: (SWANK-BACKEND:CALL-WITH-DEBUGGER-HOOK #<FUNCTION SWANK:SWANK-DEBUGGER-HOOK> #<CLOSURE (LAMBDA #) {BD1FD9D}>)
 51: ((LAMBDA ()))
 52: (SWANK::CALL-WITH-BINDINGS ((*STANDARD-OUTPUT* . #) (*STANDARD-INPUT* . #) (*TRACE-OUTPUT* . #) (*ERROR-OUTPUT* . #) (*DEBUG-IO* . #) (*QUERY-IO* . #) ...) #<CLOSURE (LAMBDA #) {BD1FDAD}>)
 53: ((FLET #:FORM-FUN-[CALL-WITH-CONNECTION]471))
 54: (SWANK::CALL-WITH-CONNECTION #<SWANK::CONNECTION {AA42549}> #<CLOSURE (LAMBDA #) {BD1FD9D}>)
 55: (SWANK::HANDLE-REQUESTS #<SWANK::CONNECTION {AA42549}> NIL)
 56: (SWANK::REPL-LOOP #<SWANK::CONNECTION {AA42549}>)
 57: ((LAMBDA ()))
 58: (SWANK::CALL-WITH-BINDINGS NIL #<CLOSURE (LAMBDA #) {AF9D08D}>)
 59: ((LAMBDA ()))
 60: ((FLET #:WITHOUT-INTERRUPTS-BODY-[BLOCK351]356))
 61: ((FLET SB-THREAD::WITH-MUTEX-THUNK))
 62: ((FLET #:WITHOUT-INTERRUPTS-BODY-[CALL-WITH-MUTEX]293))
 63: (SB-THREAD::CALL-WITH-MUTEX ..)
 64: ((LAMBDA ()))
 65: ("foreign function: #x80677DB")
 66: ("foreign function: #x805311D")
 67: ("foreign function: #x805E830")
 68: ("foreign function: #xB7FAB4FF")