%Warning-MULTITOP: t/t_multitop_sig.v:14: Multiple top level modules: 'b' and 'a'
                                        : ... Suggest see manual; fix the duplicates, or use --top-module to select top.
                   ... Use "/* verilator lint_off MULTITOP */" and lint_on around source to disable this message.
%Warning-MULTITOP: t/t_multitop_sig.v:22: Multiple top level modules: 'c' and 'b'
%Error: Exiting due to