Default value for a task variable?

[khamburg]

I want to find a simple way to initialize a task variable. I tried to do it with a script:

<task-node name="tNode">
 <task name="tName">
 <event type="task-create">
 <script>
 contextInstance.variables.a = "yes";
 </script>
 </event>
 <controller>
 <variable name="a" access="read,write">
 </variable>
 </controller>
 </task>
</task-node>

but I get an error:

SEVERE: Error taking transition: Sourced file: inline evaluation of: ``contextInstance.variables.a= "yes";'' : Field access: bsh.ReflectError: No such field: a

I tried all the supported event types: {task-create|task-start|task-assign|task-end} and got the same result.