How to filter an array and sum the total

i have a JSON data as follows:

{

   "transactions": [

     {"city":"toronto", "sales":1234.42, "account":"john"},

     {"city":"toronto","sales":4502.54, "account":"mary"},

     {"city":"montreal","sales":520.04, "account":"tony"}

  ]

}

In Word, i create a table to show sales, filtered for "toronto" using the "discard-if" expression. 

I've added a table footer that i want to perform a "sum" on "sales" using JSONata's $sum function: Numeric aggregation functions · JSONata

The sum fuction operates on all elements of the array instead of the filtered array. Is there a way to dynamically sum on the filtered array? Is this supported?

Thanks for any guidance.