New symbol to count rows matched in X[Y] #2124

I was expecting these to give the same result:

X = data.table(id = "a", key = "id")
X["b", .N]              
# 1
X["b", .N, by=.EACHI][, sum(N)] 
# 0

Thinking it through, I understand why they don't, but it would be nice to have a simple way of getting a count of matches without always having to type .N, nomatch=0 (or nomatch=NULL after that the planned change, #857).

So maybe X[Y, .NMATCH], a special symbol that like by=.EACHI is only available in a join?

My use case is a table of (item, task) pairs, where my workflow involves skipping chunks of code iff I have no items associated with a task. So I parse that table like do_thistask = X["thistask", .N > 0L] but now realize I need nomatch=0/NULL.