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DFDL has a feature known by the dfdl:outputValueCalc (OVC for short) property. This property holds an expression in DFDL's expression language, and this expression typically refers to elements that are later in the infoset than the element whose declaration carries the OVCOVC is a tremendously powerful feature in DFDL which enables a DFDL schema to truly capture inter-dependencies between elements, typically when one element stores the length of another element, but where these two elements are not adjacent in the representation. For example, a length field may appear in a header part of the data, and the part of the data whose length is given in that header, is represented much later in the data stream.

The Daffodil unparser is designed to support streaming behavior - the infoset arrives as a series of infoset events generated by a Daffodil InfosetInputter. Daffodil attempts to stream output data to the output stream without waiting for the entire infoset to arrive. Ideally, once an infoset element's start and end events have arrived the element's representation could be written to the output stream. When an infoset element has been unparsed, then in principle it can be pruned from the infoset and its memory recovered. This enables a very large infoset to be unparsed using only a finite memory footprint that is much smaller than it would be if it held the entire infoset.

TBD: A figure (or several) right here would be good illustrating incoming infoset events, incremental construction of the infoset tree, with simultaneous unparsing of these infoset elements to the output, and pruning of the unparsed elements.good illustrating incoming infoset events, incremental construction of the infoset tree, with simultaneous unparsing of these infoset elements to the output, and pruning of the unparsed elements.

However, the OVC feature complicates streaming behaviorHowever, the OVC feature complicates this. OVC is a tremendously powerful feature in DFDL which enables a DFDL schema to truly capture inter-dependencies between elements, typically when one element stores the length of another element, but where these two elements are not adjacent in the infoset. For example, a length field may appear in a header part of the data, and the part of the data whose length is given in that header, is represented much later in the data stream.

OVC elements typically don't appear in the stream of infoset events. In order to support data parsing and unparsing, they are tolerated, but ignored, if they appear in the infoset events, and the values are recomputed. For purposes of this discussion we'll assume that no events appear in the infoset events corresponding to OVC elements. 

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