@misc{indiciaedc29f66995a9,
  title = {The Secondary Boundary Operator.},
  author = {J H Whitehead},
  year = {1950},
  doi = {10.1073/pnas.36.1.55},
  url = {https://pubmed.ncbi.nlm.nih.gov/16588949/},
  note = {Source identifier: 16588949}
}