Tests whether a 2 x n matrix represents a rooted tree in the
format accepted by the
a 2 x n matrix.
In the accepted format, the columns of the integer matrix represent the edges of the tree, with the entry in the first row being closer to the root. The leaves have to be the smallest integers of the matrix. If an edge has a smaller column index than a second one, it may not be on the path from the second edge to the root (the order of the columns has to be compatible with the partial order of the edges of the tree).