Bug check 1901

[ProgramCrafter]

I've been developing a proof, one of its steps would not unify and I finally realized that I had a syntax error in definition. Original .mm file:

$( We build upon all known math theory. $)
$[ metamath/set.mm $]

$( A sequence of type ~ S has type ` Word S `. $)

$( Extend class notation to include the set of increasing sequences. $)
$c UpWord $.
cupword   $a class UpWord S $.

${
    df-upword $a |- UpWord S = { w | ( w e. Word S /\ A. k e. ( 0 ..^ ( # ` w - 1 ) ) ( w ` k < w ` ( k + 1 ) ) ) } $.
    
    upwordnul $p |- (/) e. UpWord S $= $.
$}

(the syntax error itself: # \ w` should've been surrounded by parentheses)

After developing most of the proof, I noticed the problem and tried to locally rewrite the definition. The bug appeared.

MM-PA> sh new /lemmon
 17 wrd0         $p |- (/) e. Word S
 21 elsb1        $p |- ( [ (/) / $35 ] $35 e. Word S <-> (/) e. Word S )
 22 17,21 mpbir  $p |- [ (/) / $35 ] $35 e. Word S
 50 ?            $? |- ( $35 = (/) -> ( # ` $35 - 1 ) <_ 0 )
 64 nn0ssz       $p |- NN0 C_ ZZ
 68 ?            $? |- ( $35 = (/) -> $35 e. Word S )
 69 ?            $? |- ( $35 e. Word S -> # ` $35 e. NN0 )
 70 68,69 syl    $p |- ( $35 = (/) -> # ` $35 e. NN0 )
 71 64,70 sselid  $p |- ( $35 = (/) -> # ` $35 e. ZZ )
 73 1zzd         $p |- ( $35 = (/) -> 1 e. ZZ )
 74 71,73 zsubcld  $p |- ( $35 = (/) -> ( # ` $35 - 1 ) e. ZZ )
 75 0z           $p |- 0 e. ZZ
 76 74,75 jctil  $p |- ( $35 = (/) -> ( 0 e. ZZ /\ ( # ` $35 - 1 ) e. ZZ ) )
 79 fzon         $p |- ( ( 0 e. ZZ /\ ( # ` $35 - 1 ) e. ZZ ) -> ( ( # ` $35 - 1 ) <_ 0 <-> ( 0 ..^ ( # ` $35 - 1 ) ) = (/) ) )
 80 76,79 syl    $p |- ( $35 = (/) -> ( ( # ` $35 - 1 ) <_ 0 <-> ( 0 ..^ ( # ` $35 - 1 ) ) = (/) ) )
 81 50,80 mpbid  $p |- ( $35 = (/) -> ( 0 ..^ ( # ` $35 - 1 ) ) = (/) )
 90 neq0         $p |- ( -. ( 0 ..^ ( # ` $35 - 1 ) ) = (/) <-> E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) )
 91 90 bicomi    $p |- ( E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) <-> -. ( 0 ..^ ( # ` $35 - 1 ) ) = (/) )
 92 91 con2bii   $p |- ( ( 0 ..^ ( # ` $35 - 1 ) ) = (/) <-> -. E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) )
 93 92 biimpi    $p |- ( ( 0 ..^ ( # ` $35 - 1 ) ) = (/) -> -. E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) )
 94 81,93 syl    $p |- ( $35 = (/) -> -. E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) )
 97 imnan        $p |- ( ( $35 = (/) -> -. E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ) <-> -. ( $35 = (/) /\ E. $110 $110 e. ( 0 ..^ ( # `
                                                                                                                             $35 - 1 ) ) ) )
 98 94,97 mpbi   $p |- -. ( $35 = (/) /\ E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) )
 99 98 bifal     $p |- ( ( $35 = (/) /\ E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ) <-> F. )
100 99 biimpi    $p |- ( ( $35 = (/) /\ E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ) -> F. )
102 falim        $p |- ( F. -> A. $110 ( $35 ` $110 < $35 ` ( $110 + 1 ) ) )
103 100,102 syl  $p |- ( ( $35 = (/) /\ E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ) -> A. $110 ( $35 ` $110 < $35 ` ( $110 + 1 ) ) )
104 103 ex       $p |- ( $35 = (/) -> ( E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) -> A. $110 ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) )
108 19.38        $p |- ( ( E. $110 $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) -> A. $110 ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) -> A. $110 ( $110 e. (
                                                                          0 ..^ ( # ` $35 - 1 ) ) -> ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) )
109 104,108 syl  $p |- ( $35 = (/) -> A. $110 ( $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) -> ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) )
113 df-ral       $a |- ( A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) <-> A. $110 ( $110 e. ( 0 ..^ ( # ` $35 -
                                                                                            1 ) ) -> ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) )
114 109,113 sylibr  $p |- ( $35 = (/) -> A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) )
115 114 ax-gen   $a |- A. $35 ( $35 = (/) -> A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) )
119 sb6          $p |- ( [ (/) / $35 ] A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) <-> A. $35 ( $35 = (/) -> A.
                                                                   $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) )
120 115,119 mpbir  $p |- [ (/) / $35 ] A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) )
121 22,120 pm3.2i  $p |- ( [ (/) / $35 ] $35 e. Word S /\ [ (/) / $35 ] A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1
                                                                                                                                       ) ) )
126 sban         $p |- ( [ (/) / $35 ] ( $35 e. Word S /\ A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) <-> ( [
                     (/) / $35 ] $35 e. Word S /\ [ (/) / $35 ] A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) )
127 121,126 mpbir  $p |- [ (/) / $35 ] ( $35 e. Word S /\ A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) )
131 df-clab      $a |- ( (/) e. { $35 | ( $35 e. Word S /\ A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) } <->
                               [ (/) / $35 ] ( $35 e. Word S /\ A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) )
132 127,131 mpbir  $p |- (/) e. { $35 | ( $35 e. Word S /\ A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) }
136 df-upword    $a |- UpWord S = { $35 | ( $35 e. Word S /\ A. $110 e. ( 0 ..^ ( # ` $35 - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) }
137 132,136 eleqtrri  $p |- (/) e. UpWord S
MM-PA> le st 136
With what math symbol string? $a |- UpWord S = { $35 | ( $35 e. Word S /\ A. $110 e. ( 0 ..^ ( ( # ` $35 ) - 1 ) ) ( $35 ` $110 < $35 ` ( $110 + 1 ) ) ) }
?BUG CHECK:  *** DETECTED BUG 1901

To get technical support, please open an issue 
(https://github.com/metamath/metamath-exe/issues) with the
detailed command sequence or a command file that reproduces this bug,
along with the source file that was used.  See HELP OPEN LOG for help on
recording a session.  See HELP SUBMIT for help on command files.  Search
for "bug(1901)" in the m*.c source code to find its origin.
If earlier errors were reported, try fixing them first, because they
may occasionally lead to false bug detection