Toby's sour cherry shrub

Ingredients

Rinse cherries and trim any rotted parts
Put cherries (unpitted) into a large nonreactive pot
Mash cherries for several minutes
Pour in enough apple cider vinegar to cover the cherries
Cover and let rest for one week, stirring daily (it might smell a bit)
At the end of the week, stir in sugar and gently boil for one hour
Let cool slightly and strain through a cheesecloth-lined fine-mesh sieve
Test by adding a small amount to iced club soda; add more sugar or cherries if needed
Cool completely and refrigerate (it should keep indefinitely)