message: allow `timescale 1ns / 10 ps
  > space between the number and the time unit
diff: @@ -1403,7 +1403,7 @@ simple_hierarchical_identifier : simple_hierarchical_branch ( '.' Escaped_identi
  specparam_identifier : identifier ;
  Simple_identifier : [a-zA-Z_] [a-zA-Z0-9_$]* ;
  Dollar_Identifier : '$' [a-zA-Z0-9_$] [a-zA-Z0-9_$]* ;
  -Time_Identifier : [0-9]+ [mnpf]'s';
  +Time_Identifier : [0-9]+ White_space* [mnpf]'s';
  system_function_identifier : Dollar_Identifier ;
  system_task_identifier : Dollar_Identifier ;
  task_identifier : identifier ;
  