Clarifications added to the problem statement regarding river basin definition, river intersection restrictions and input precision.

Tests that were invalid according to the clarifications have been fixed or replaced. Tests 9 and 13 contained overlapping segments making the river flow the same path back and forth; the tests have been replaced with new tests. Tests 15, 16, 18, 19 contained rivers with self-intersections; the tests have been corrected. Test 23 contained a river with a mouth lying on the segment of the other river and didn’t count as tributary; the test has been corrected, this situation is no longer allowed in the tests.

New tests have been added. In particular, tests with more than 2 rivers in a basin have been introduced.

All solutions have been rejudged. 106 authors lost, and 12 other authors gained the solved status for the problem.