%Error: t/t_string_add_bad.v:13:9: Operator ADD is not legal on string data types (IEEE 1800-2023 6.16)
                                 : ... note: In instance 't'
                                 : ... Suggest to concatenate strings use '{LHS, RHS, ...}'
   13 |       s += $sformatf(" a%0d", a);   
      |         ^~
        ... See the manual at https://verilator.org/verilator_doc.html?v=latest for more assistance.
%Error: t/t_string_add_bad.v:14:13: Operator ADD is not legal on string data types (IEEE 1800-2023 6.16)
                                  : ... note: In instance 't'
                                  : ... Suggest to concatenate strings use '{LHS, RHS, ...}'
   14 |       s = s + s;   
      |             ^
%Error: Exiting due to