  Defects4J has enabled numerous software testing and debugging research work
since its introduction. A large part of its contribution, and the resulting
popularity, lies in the clear separation and distillation of the root cause of
each individual test failure based on careful manual analysis, which in turn
allowed researchers to easily study individual faults in isolation. However, in
a realistic debugging scenario, multiple faults can coexist and affect test
results collectively. Study of automated debugging techniques for these
situations, such as failure clustering or fault localisation for multiple
faults, would significantly benefit from a reliable benchmark of multiple,
coexisting faults. We search for versions of Defects4J subjects that contain
multiple faults, by iteratively transplanting fault-revealing test cases across
Defects4J versions. Out of 326 studied versions of Defects4J subjects, we
report that over 95% (311 versions) actually contain from two to 24 faults. We
hope that the extended, multi-fault Defects4J can provide a platform for future
research of testing and debugging techniques for multi-fault programs.
