Better handling of unrecognized JSL substitution operators

Need to think about how to handle unrecognized JSL substitution operators.

The batch spec (JSR 352) defines 4 JSL substitution operators:

1. jobParameters - specifies to use a named parameter from the job parameters.
2. jobProperties - specifies to use a named property from among the job's properties.
3. systemProperties - specifies to use a named property from the system properties.
4. partitionPlan - specifies to use a named property from the partition plan of a partitioned step.

How to handle unrecognized substitution operators, such as

#{myData['foo']}

JBeret currently issues a warning, and returns null (which will resolve to an empty xml string during job xml loading process). Is it better to just return the property expression verbatim?