struct::set diff -> struct::set difference shortform ensemble command only works if critcl available

src/modules/textblock-999999.0a1.0.tm

@@ -2839,7 +2839,7 @@ namespace eval textblock {
 
         #boxlimits used for partial borders in table generation
         set all_exact_boxlimits [list vll vlr hlt hlb tlc blc trc blc]
-        set unspecified_limits [struct::set diff $all_exact_boxlimits $exact_boxlimits]
+        set unspecified_limits [struct::set difference $all_exact_boxlimits $exact_boxlimits]
         foreach lim $unspecified_limits {
             switch -- $lim {
                 vll {