The enter() method is plumbing used by the run(), bind(), andintercept() methods to set the active domain. It sets domain.active andprocess.domain to the domain, and implicitly pushes the domain onto the domain stack managed by the domain module (see exit for details on the domain stack). The call to enter() delimits the beginning of a chain of asynchronous calls and I/O operations bound to a domain.

Calling enter() changes only the active domain, and does not alter the domain itself. enter() and exit() can be called an arbitrary number of times on a single domain.