  In this short note we observe that a result of Eliashberg and Polterovitch
allows to use the doubly slice genus as an obstruction for a Legendrian knot to
be a slice of a concordance from the trivial Legendrian knot with maximal
Thurston-Bennequin invariant to itself. This allows to obstruct concordances
from the Pretzel knot P(3,-3,-m) when m > 3 to the unknot. Those examples are
of interest because the Legendrian contact homology algebra cannot be used to
obstruct such a concordance.
