Using only implication is problematic if we add negation to chain. In
such case its meaning isn't that it implies negative value, but that it
doesn't imply. That is problem if we use use negative dependencies. New
approach uses equivalency instead of implications. We used equivalency
originally because formulas added to sat solver as dependencies of one
package would introduced limitation on package even if original package
isn't selected and that is wrong. Now instead of that we use formulas
activators. It's variable that ensures that if it isn't set than
formula is always true. Activator is package group or candidate for
which given dependency is build. This way we are able to ignore
dependency formulas that don't have to be activated because we don't
choose that package.