This is an interesting chunk of code, and introduced me to yet another fascinating computer person. Ralph Griswold's articles were great to skim through.

Oh right, the code. I get errors about uninitialized variables at line 162 unless I explicitly check for the existence of a key $value in %preference_table:

if (exists $precedence_table{$value}) {
    my $precedence = ( $n + $precedence_table{$value} );
    $expression_list[$i] = [ { $key => $$_{$key} }, $precedence ];
}
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