github.com/benhoyt/goawk@v1.8.1/testdata/t.delete3 (about)

     1  { x[$1] = $1
     2    delete x[$1]
     3    n = 0
     4    for (i in x) n++
     5    if (n != 0)
     6  	print "error", n, "at", NR
     7  }