Add ability to count the items in a task queue

task.maude

@@ -24,10 +24,14 @@ mod TASK-QUEUE is
   op mt : -> TaskQueue .
   op __ : TaskQueue TaskQueue -> TaskQueue [ctor assoc id: mt] .
   op insert : TaskQueue Task -> TaskQueue [left id: mt] .
+  op count : TaskQueue -> Nat .
 
   var Q : TaskQueue . var J K : Task .
   vars N M : Oid . vars T1 T2 : Nat .
 
   ceq insert(Q J, K) = insert(Q, K) J if K < J .
   eq insert(Q J, K) = Q J K [owise] .
+
+  eq count(mt) = 0 .
+  eq count(Q J) = s(count(Q)) .
 endm