  We point out that the computation of true \emph{proof} and \emph{disproof}
numbers for proof number search in arbitrary directed acyclic graphs is
NP-hard, an important theoretical result for proof number search. The proof
requires a reduction from SAT, which demonstrates that finding true
proof/disproof number for arbitrary DAG is at least as hard as deciding if
arbitrary SAT instance is satisfiable, thus NP-hard.
