def METHOD_NAME(prev_atoms, atoms):
  prevCA, prevC, thisN, thisCA = None, None, None, None
  if (prev_atoms is not None):
    for atom in prev_atoms:
      if (atom.name == " CA "): prevCA = atom
      if (atom.name == " C  "): prevC = atom
  if (atoms is not None):
    for atom in atoms:
      if (atom.name == " N  "): thisN = atom
      if (atom.name == " CA "): thisCA = atom
  return prevCA, prevC, thisN, thisCA