  In 1993, two proofs of the persistence of regularity of the boundary of a
classical vortex patch for the 2D Euler equations were published, one by Chemin
(announced in 1991) the other by Bertozzi and Constantin. Chemin, in fact,
proved a more general result, extending it further in 1995 showing, roughly,
that vorticity initially having discontinuities only in directions normal to a
family of vector fields that together foliate the plane continue to be so
characterized by the time-evolved vector fields. A different, four-page
"elementary" proof of Chemin's 1993 result was published in 1994 by Ph.
Serfati, who also gave a fuller characterization of the velocity gradient's
regularity. We give a detailed version of Serfati's proof along with an
extension of it to a family of vector fields that reproduces the 1995 result of
Chemin.
