First input is T, a triplet list of indices. Second input is i, a single index (positive integer). The goal of this function is to find and return the indices of the rows in the list which contain this particular index. Output format is indifferently a column or a row vector.
For example if inputs are
T = [1 2 3;... 1 3 4;... 1 4 2;... 2 3 4]
i = 2
the output is the vector
row_idx = [1 3 4]
since 2 is contained in rows number 1, 3, and 4 of T. If the index is not in the list, the function must of course return the empty set. Each index is at most contained once per row / triplet.