  We describe SeCaV, a sequent calculus verifier for first-order logic in
Isabelle/HOL, and the SeCaV Unshortener, an online tool that expands succinct
derivations into the full SeCaV syntax. We leverage the power of Isabelle/HOL
as a proof checker for our SeCaV derivations. The interactive features of
Isabelle/HOL make our system transparent. For instance, the user can simply
click on a side condition to see its exact definition. Our formalized soundness
and completeness proofs pertain exactly to the calculus as exposed to the user
and not just to some model of our tool. Users can also write their derivations
in the SeCaV Unshortener, which provides a lighter syntax, and expand them for
later verification. We have used both tools in our teaching.
