MDB_DEF_HISTLEN
mdb.m_histlen = MDB_DEF_HISTLEN;
ssize_t newhalloc = cmd->cmd_halloc + MDB_DEF_HISTLEN;
cmd->cmd_halloc = MDB_DEF_HISTLEN < mdb.m_histlen ?
MDB_DEF_HISTLEN : mdb.m_histlen;