Maintain symbolic links determining default commands

update-alternatives creates, removes, maintains and displays
information about the symbolic links comprising the alternatives
system. It is possible for several programs fulfilling the same or
similar functions to be installed on a single system at the same time.
For example, many systems have several text editors installed at once.
This gives choice to the users of a system, allowing each to use a
different editor, if desired, but makes it difficult for a program to
make a good choice of editor to invoke if the user has not specified a
particular preference.