# Data file format, a pickled dict, with keys:
#
# collector: a string identifying the collecting software
# lines: a dict mapping filenames to lists of line numbers executed:
#   { 'file1': [17,23,45],  'file2': [1,2,3], }
# partial_lines: a dict mapping filenames to dicts mapping line numbers to
#   partial info. Partial info is a tuple, exec, poss, explanation?
#   Fully-executed lines can be omitted, as well as fully-non-executed lines.
#   { 'file1': { 1: (4,8,'a myfunc'),  2: (99,100,''), }, }
#