It seems that $calc returns 0 for all invalid calculations, not just for divide by zero errors.

//echo -a $calc(1 / 0)
//echo -a $calc(1 / !)
//echo -a $calc(1 / -)
//echo -a $calc(1 / banana)
//echo -a $calc(1 /x 1)
//echo -a $calc(1 /! 1)
etc..


although... some just return the first value and ignore the invalid parts.

//echo -a $calc(1 +d 1)
//echo -a $calc(2 +d 4)
//echo -a $calc(4 -d 1)

-genius_at_work