Quickly identifies edges that are 'descended' from a particular edge in a tree
DescendantEdges(edge, parent, child, nEdge = length(parent))number of the edge whose child edges are required
the first column of the edge matrix of a tree of class phylo, i.e. tree$edge[, 1]
the second column of the edge matrix of a tree of class phylo, i.e. tree$edge[, 2]
number of edges (calculated from length(parent) if not supplied)
a logical vector stating whether each edge in turn is a descendant of the specified edge (or the edge itself)