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[...]
cd obj_opt && make  TGT=../../verilator_bin -f ../Makefile_obj
make[3]: Entering directory `/build/buildd-verilator_3.805-1-hppa-HEPYtD/verilator-3.805/src/obj_opt'
      Compile flags:  g++ -I/usr/include -MMD -I. -I.. -I.. -I../../include -DYYDEBUG -O -DDEFENV_SYSTEMC="" -DDEFENV_SYSTEMC_ARCH="" -DDEFENV_SYSTEMPERL="" -DDEFENV_SYSTEMPERL_INCLUDE="" -DDEFENV_VERILATOR_ROOT="/usr/share/verilator"
make[3]: Leaving directory `/build/buildd-verilator_3.805-1-hppa-HEPYtD/verilator-3.805/src/obj_opt'
make[2]: Leaving directory `/build/buildd-verilator_3.805-1-hppa-HEPYtD/verilator-3.805/src'
make[2]: Entering directory `/build/buildd-verilator_3.805-1-hppa-HEPYtD/verilator-3.805/test_c'
/usr/bin/perl /build/buildd-verilator_3.805-1-hppa-HEPYtD/verilator-3.805/test_c/../bin/verilator        --cc -f /build/buildd-verilator_3.805-1-hppa-HEPYtD/verilator-3.805/test_c/../test_v/input.vc top.v --trace
%Error: Internal Error: ../V3Ast.cpp:491: Node has no back, already unlinked?
%Error: Internal Error: See the manual and http://www.veripool.org/verilator for more assistance.
%Error: Command Failed /build/buildd-verilator_3.805-1-hppa-HEPYtD/verilator-3.805/test_c/../verilator_bin --cc -f /build/buildd-verilator_3.805-1-hppa-HEPYtD/verilator-3.805/test_c/../test_v/input.vc top.v --trace
make[2]: *** [prep] Error 9
make[2]: Leaving directory `/build/buildd-verilator_3.805-1-hppa-HEPYtD/verilator-3.805/test_c'
make[1]: *** [test_c] Error 2
make[1]: Leaving directory `/build/buildd-verilator_3.805-1-hppa-HEPYtD/verilator-3.805'
dh_auto_test: make -j1 test returned exit code 2
make: *** [build] Error 29
dpkg-buildpackage: error: debian/rules build gave error exit status 2