'parallel' statement: next steps

tests/parallel/tinvalid_counter_usage.nim

@@ -0,0 +1,26 @@
+discard """
+  errormsg: "invalid usage of counter after increment"
+  line: 21
+"""
+
+import threadpool
+
+proc f(a: openArray[int]) =
+  for x in a: echo x
+
+proc f(a: int) = echo a
+
+proc main() =
+  var a: array[0..30, int]
+  parallel:
+    spawn f(a[0..15])
+    spawn f(a[16..30])
+    var i = 0
+    while i <= 30:
+      inc i
+      spawn f(a[i])
+      inc i
+      #spawn f(a[i+1])
+      #inc i  # inc i, 2  would be correct here
+
+main()