  Formation or destruction of hyperbolic chaotic attractor under parameter
variation is considered with an example represented by Smale--Williams solenoid
in stroboscopic Poincar\'{e} map of two alternately excited non-autonomous van
der Pol oscillators. The transition occupies a narrow but finite parameter
interval and progresses in such way that periodic orbits constituting a
"skeleton" of the attractor undergo saddle-node bifurcation events involving
partner orbits from the attractor and from a non-attracting invariant set,
which forms together with its stable manifold a basin boundary of the
attractor.
