I have an ivy.xml file with some confs like so:
<conf name="abstract" visibility="private" />
<conf name="concrete" extends="abstract" visibility="public" />

Note that I have a public Ivy conf extending a private one. Can anyone say
if I'm doing something valid or invalid--and then explain conceptually why?

I suspect the answer is invalid because when I attempt to do an Ivy resolve
on this published Ivy module, I get an error message like so:
configuration not public in net.nilistics#test-module;1.0: 'abstract'. It
was required from net.nilistics#test-module-caller;working abstract

I've traced this error message to this passage in the private
*handleConfiguration
*method of *IvyNode*:
                } else if (shouldBePublic && !isRoot()
                        && c.getVisibility() !=
Configuration.Visibility.PUBLIC) {
                    confsToFetch.remove(conf);
                    if (isConfRequiredByMergedUsageOnly(rootModuleConf,
conf)) {
                        Message.verbose(
                            "configuration required by evicted revision is
not visible in "
                            + "selected revision. skipping " + conf + " in "
+ this);
                    } else {
                        problem = new RuntimeException("configuration not
public in " + this + ": '"
                            + c + "'. It was required from " + parent + " "
+ parentConf);
                    }
                    return false;
                }

I am using Ivy 2.1.0-rc2.