bug check 1710

[GinoGiotto]

File that generates the bug:

$c A $.

ax1 $ A $.

${
  in.1 $e A $.
  in $a A $.
$}

th1 $p A $= ax1 in $.

Metamath-exe interface:

Metamath - Version 0.199.pre 29-Jan-2022      Type HELP for help, EXIT to exit.
MM> set scroll continuous
Continuous scrolling is now in effect.
MM> read issue168.mm /verify
Reading source file "issue168.mm"... 78 bytes

?Error on line 3 of file "issue168.mm":
ax1 $ A $.
      ^
A keyword must be followed by white space.

78 bytes were read into the source buffer.

?Error on line 3 of file "issue168.mm" at statement 1, type "$c":
ax1 $ A $.
    ^^
Expected "$c", "$v", "$e", "$f", "$d", "$a", "$p", "${", or "$}" here.

?Error on line 3 of file "issue168.mm" at statement 1, type "$c":
ax1 $ A $.
        ^^
Expected "$c", "$v", "$e", "$f", "$d", "$a", "$p", "${", or "$}" here.
The source has 6 statements; 1 are $a and 1 are $p.

?Error on line 3 of file "issue168.mm" at statement 2, type "${":
ax1 $ A $.
^^^
A label isn't allowed for this statement type.
SET EMPTY_SUBSTITUTION was turned ON (allowed) for this database.
0 10%  20%  30%  40%  50%  60%  70%  80%  90% 100%
..................................................?BUG CHECK:  *** DETECTED BUG 1710

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(1710)" 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

Press S  to step to next bug, I  to ignore further bugs,
or A  to abort program:  S

Warning!!!  A bug was detected, but you are continuing anyway.
The program may be corrupted, so you are proceeding at your own risk.

?BUG CHECK:  *** DETECTED BUG 2102
Press S  to step to next bug, I  to ignore further bugs,
or A  to abort program:  S


4 errors were found.
MM>

The bug occurs when read issue168.mm has either the /VERIFY option active or when the verify proof * command follows.

EDIT: The following simpler file produces the same bug:

$c A $.

ax1 $ A $.

${

th1 $p A $= ax1 $.