Egon Willighagen, at Uppsala University, has pioneered the use of object properties from CiTO, the Citation Typing Ontology, to characterize bibliographic citations in CiteULike, the free service for managing and discovering scholarly references. Indeed, it was his use case that persuaded me of the need to generalize CiTO to include indirect citations.
For example, in documenting papers that use the Chemistry Development Kit (CDK), Egon records the following paper by Guha (2007) (Egon’s CiteULike reference 7901082):
He tags that Guha paper, using the CiTO tag cito:usedMethodIn, as using the CDK method described in a paper by Steinbeck et al. (2003) (Egon’s CiteULike reference 423382):
However, the original paper by Guha that has been tagged by Egon does not cite Steinbeck et al. (2003) directly in its reference list. Rather, it contains a reference to a later paper on CDK by Steinbeck et al. (2006), entitled “Recent developments of the Chemistry Development Kit (CDK)” (Egon’s CiteULike reference 1073448):
. . . , a paper that Egon tags as extending the original 2003 paper describing CDK, using the property cito:extends.
Thus the Guha paper cites Steinbeck et al. (2003) only indirectly, via Steinbeck et al. (2006). For this reason, the scope of CiTO was extended to include such indirect citations. The description of CiTO now reads:
“The purpose of the CiTO Ontology is to enable characterization of the nature or type of citations, both factually and rhetorically. The citations characterized may be either direct and explicit (as in the reference list of a journal article), indirect (e.g. a citation to a more recent paper by the same research group on the same topic), or implicit (e.g. as in artistic quotations or parodies, or in cases of plagiarism).”