+ o drop.tip() sometimes shuffled node labels (thanks to Rebecca
+ Best for the report).
+
+ o drop.tip(phy, "") returned a tree with zero-length tip labels:
+ it now returns the tree unchanged (thanks to Brian Anacker for
+ the report).
+
+ o plot.phylo() made R crash if the tree has zero-length tip
+ labels: it now returns NULL (thanks again to Brian Anacker).