grammar 
   t19 = t18 t18

   boundary = ~    -- not used; defined so we can see compilation results for ~
   t0 = "x"
   t1 = t0 t0
   t2 = t1 t1
   t3 = t2 t2
   t4 = t3 t3
   t5 = t4 t4
   t6 = t5 t5
   t7 = t6 t6
   t8 = t7 t7
   t9 = t8 t8
   t10 = t9 t9
   t11 = t10 t10
   t12 = t11 t11
   t13 = t12 t12
   t14 = t13 t13
   t15 = t14 t14
   t16 = t15 t15
   t17 = t16 t16
   t18 = t17 t17
end
