Dependencies are a graph, not a tree. Say z depends on x and y, which version of foo do you use? Requiring a single version makes certain things a lot easier t

Dependencies are a graph, not a tree. Say z depends on x and y, which version of foo do you use? Requiring a single version makes certain things a lot easier to deal with, because not all dependencies are actually inherited by another. If foo is an executable then sure, you can have multiple versions and it's no issue. If foo is a library then you might need to fail because there is no solution that works. Or you might not, it depends.