The linear network L
must be an acyclic graph (i.e. must not contain any loops) so that it
can be interpreted as a tree.
The result of f <- branchlabelfun(L, root) is
a function f which gives,
for each location on the linear network L,
the tree branch label at that location.
Tree branch labels are explained in treebranchlabels.
The result f also belongs to the class "linfun".
It can be called using several different kinds of data,
as explained in the help for linfun.
The values of the function are character strings.