LII remarks on identifiers for legislation

The following was written (by Dave Shetland and Tom Bruce) as a position statement for the Workshop on Legislative XML associated with the 2007 JURIX conference in Leiden:

Resilience in identifier design, and some thoughts on granularity

The Legal Information Institute (LII) has a great deal of experience with
processing the United States Code (USC), as well as US Supreme Court
opinions and various other legislative and case law resources. Here we
address two closely related questions that confront the XML and Legislation
Workshop associated with Jurix 2007, having to do with our use of
identifiers and the means by which we determine granularity.

The USC consists of about 50,000 sections containing the statutes as
codified, together with about 8,000 section-size container elements like
Chapter. All authoring and revision is done by the US Congress and its
Office of the Law Revision Counsel before the LII receives it. It is
transmitted in a specialized, highly modal encoding used for typesetting.
Decisions about appropriate granularity and identifier schemes -- which in
any case are closely related -- have been driven by the realities and
contingencies of an automated filtering process used to create an XML
version of the US Code from this typesetting data. This is perhaps an
easier task than supporting the full life-cycle of drafting, passage, and
promulgation, but suffers from noisier and occasionally mis-structured
data, over which we have little initial control. Some aspects of that
processing system, of particular interest to identifier design and usage,
are listed here:

(1) The initial XML schema emphasizes retention of source data, much of
which will not be understood well at the outset, or perhaps ever, but all
of which is considered a potentially valuable resource.
(2) The system, including the XML, supports an incremental improvement
approach to citation reference detection and associated link generation.
Specifically, the system needs to determine whether a potential cross-
reference proposed by the automatic linker is valid.
(3) Xpath specifications can be safely used for structured access to
elements without identifiers within small structural units, such as certain
paragraph fragments and references. This would not be true of larger
(4) XML fragment size is currently that of USC section, which allows for
efficient file sizes (one fragment per file). The file sizes range over
three orders of magnitude, corresponding to variations in USC section bulk
and complexity.
(5) Element instance identifiers are used. These distill and normalize
primary identifying data, and represent deliberate compromise between the
extremes of mindless abstraction and mindless recitation. Thus, instead of
the literal text citation section number of 12_USC_1749bbb-10a, we see it
padded out to usc_sec_12_00001749-bbb010a for ease of collation, but not
converted into a hash code requiring data base reflection to yield any
semantic value.

Data-set independent identifiers should be added to the model to allow for
tracking of arbitrary movement of content. We have yet to add point-in-time
historical tracking, and we have no role in the editing process. A system-
defined identifier facility will be needed at some time, but has not yet
been implemented. When it is used, it will answer the single primary ID
expectation. But other identifiers, including data-derived ones such as we
now employ as primary, would still be needed for portability, archiving,
efficiency of associating meta-data with structural units, convenience of
programming, and compatibility with real-world, legacy identifier systems.
Existing print-based citation systems and structural labels that have
become terms of art within the profession (eg. 501(c)(3) nonprofit) are
particularly important in this respect.

These factors have led us to designs in which semantically-neutral
identifiers are used internally, and semantically-laden identifiers
provided externally by means of a software translation layer. This allows
association of a particular fragment with multiple identifier schemes.
While this is a less important capability for legislation than it is for
caselaw, where a particular case may have many citations over its lifetime,
it is nevertheless extremely useful.


For us, granularity has often been a matter of specifying the smallest
structural unit that we need to make externally addressable. We have
deliberately blurred the dual nature of identifiers as labels. At times
they are pinpoint destinations within a text stream; at others, they are
handles by which structural units can be retrieved. The smallest unlabeled
block of text that we treat as a container is the paragraph. Were we to
support the drafting and revision of legislation, we might need finer
granularity. But it seems to us that there is a reasonable lower limit;
rather than assigning identifiers to bytes or tokens, it makes sense to
turn fine-grained editing over to subsystems that take (eg.) paragraphs as
input and produce altered paragraphs as output.


The task of presenting statutory text derived from a legacy data format, in
a manner that retains authority, is far different from the task of defining
a new approach to authoring the source data. Techniques and devices for
discovery of existing material in a data set (reflection, etc.) as well
as for resolution of addressing ambiguities are needed by both kinds of
support systems. But a variety of identifier schemes may be expected in
support of the different systems. In fact they will co-exist within any
system supporting migration from legacy data. Any single-identifier mandate
must be considered external to the mandates inherent in the base data sets,
which must be allowed their own identifier schemes, if only recorded as
part of the routine metadata.