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This breaks the rule that when variables are evaluated, things they reference must have already been evaluated. Basically, when we delay evaluating the OVC for len we are suspending anything that depends on len having a value as well.

fn:error(...) Function needed to issue errors when unparsing

When parsing, one has the alternative of creating a dfdl:assert that tests something and fails if it doesn't hold.

However, dfdl:assert expressions aren't evaluated when unparsing; hence, a way to indicate an error in data during unparse-time expression evaluation is needed.

The XPath fn:error(...) function is suitable.

Parse-Time Forward Reference

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