The mathematics of uniqueness: a systematic study of the properties of events (e.g. subsequences of a string) that never repeat.
« Motifs appearing only once are not interesting » ~ “A basis for repeated motifs in pattern discovery and text mining”, N. Pisanti, M. Crochemore, R. Grossi, M.F. Sagot.
Pratt  allows to find all maximal motifs that cannot be made more specific without becoming unique events in the input string.
 Jonassen, Collins, Higgins. Finding flexible patterns in unaligned protein sequences. Protein Science, 4:1587-1595, 1995.