gistfile1.txt

// This does not work
S
   : LBRACE exp_s RBRACE { $$ = $2}
   ;

exp_s
   : { $$ = []; }
   |  exp SEMI exp_s { $$ = $3.unshift($1); }

exp
   : /*omitted*/

// This works   
S
   : LBRACE exp_s RBRACE { $$ = $2}
   ;

exp_s
   : { $$ = null; }
   | exp_p { $$ = $1; }
   ;
exp_p
   : exp SEMI { $$ = [$1];}
   | exp SEMI exp_p { $$ = $3.unshift($1);}
   ;

exp
   : /*omitted*/