Nested PSL SEREs are not supported

[Blebowski]

A SERE that is nested withing another SERE can't be elaborated by NVC.

Steps to reproduce:

git clone https://github.com/Blebowski/psl_with_ghdl
cd psl_with_ghdl
git checkout b514c001c934826661a9f4ff8f4415fd52112d45
cd sim
make psl_cover

I get:

nvc --std=08 --work=lib_psl_cover -a  ../src/pkg.vhd ../src/sequencer.vhd ../src/hex_sequencer.vhd
nvc --std=08 --work=lib_psl_cover -a --psl ../src/psl_cover.vhd
nvc --std=08 --work=lib_psl_cover -a tb_tops/psl_cover/testbench.vhd
nvc --std=08 --work=lib_psl_cover -e --cover tb_psl_cover
** Fatal: cannot handle PSL kind P_SERE in build_sere
    > ../src/psl_cover.vhd:37
    |
 37 |   -- psl COVER_1_c : cover {req; {{busy[=1]} && {not done[+]}}}
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
make: *** [Makefile:16: psl_cover] Error 1