[gentoo-user] slot conflict when updating portage

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

sys-apps/portage:0

How should I go about handling this?

Slot are explained somewhere as allowing multiple packages to have different versions of the same providing package.  Why should there be conflicts?  Is there a limited number of slots or something?  Why is a slot conflict a problem - each dependent package can use its own slot ...

Following this message there are a number of "paragraphs", each introduced with a line like the "sys-apps/portage:0" line, above.  Each paragraph contains multiple "clauses", apparently representing different versions of the package starting the "paragraph"

Each seems to be terminated with a status:
- argument
- installed
- ebuild scheduled for merge

Where's the problem?

There must be a problem because it goes on to say:

"It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously.  If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously."

I can solve the problem by preventing *one* of the packages from being selected?
Let's see, I have 3 such paragraphs, two with 2 clauses each and one with 6 clauses.  If I pick one, everything will be fine?

It then suggests looking at the MASKED PACKAGES section of the emerge man page.  But that has to do with experimental or development packages.  My profile is "stable" - there's no reason why I should have any of those, is there?

It goes on, but I think those are other issues which I will raise in a subsequent post.