rspec is a meta-gem, which depends on the rspec-core,
rspec-expectations and rspec-mocks gems. Each of these can be
installed separately and loaded in isolation using require. Among
other benefits, this allows you to use rspec-expectations, for
example, in Test::Unit::TestCase if you happen to prefer that style.

Conversely, if you like RSpec's approach to declaring example groups
and examples (describe and it) but prefer Test::Unit assertions and
mocha, rr or flexmock for mocking, you'll be able to do that without
having to install or load the components of RSpec that you're not
using.
