L1, L3 and L4 are visible but not L2 when viewed downward from y = infinity.
The input will contain an integer N indicating the number of test cases, where 0 < N <= 20. Each test case starts with an integer M, indicating the number of lines in the test case, where 0 < M < 100000. M lines of input then follows, each has two integers m and c, describing a line in the slope intersect form y = mx + c. The values for m and c are bounded by a 32 bit signed integer in C. There will be no vertical lines and no more than two lines intersect at the same point.
Output the lines that are visible. If the i-th line in the test case is visible, then output one line of integers m and c of the i-th line. The output should be sorted first by m then c. Print an empty line after each test case.
2 2 1 0 -1 0 2 0 4 0 2
-1 0 1 0 0 4