Transient dark energy, cosmological constant boundary crossing and dark
  energy $w(z)$ data fits
  The formalism of dark energy based on modeling speed of sound as a function
of equation of state parameter is elaborated. A specific model which allows
detailed study of cosmological constant boundary crossing is introduced and
analytical solutions for the model dynamics are obtained. It is shown how in
specific parameter regimes dark energy can be a transient phenomenon. It is
further demonstrated how the model reproduces specific features of recent fits
of dark energy $w(z)$ to observational data.
