Each developer works on their own private copy of the code base
Each developer works on his own private copy of the code base. Their work and any investigations they are undertaking are not interrupted each time other workers integrate their results into the main stream. It is important for developers to work with current versions of software subsystems to keep up with the latest enhancements, avoid running into already fixed bugs fixed elsewhere, and to avoid getting out of synch with interface changes.
This and other solutions related to source control and integration are now almost universally accepted and are common practice. This practice is usually paired with NamedStableBases to provide baselines upon which the individual can build his work.