anchor.diff:  
   // 2.2.2 Strengths
   drive_strength
  -   : (strength0 ',' strength1)
  -   | (strength1 ',' strength0)
  -   | (strength0 ',' 'highz1')
  -   | (strength1 ',' 'highz0')
  -   | ('highz0' ',' strength1)
  -   | ('highz1' ',' strength0)
  +   : '(' strength0 ',' strength1 ')'
  +   | '(' strength1 ',' strength0 ')'
  +   | '(' strength0 ',' 'highz1' ')'
  +   | '(' strength1 ',' 'highz0' ')'
  +   | '(' 'highz0' ',' strength1 ')'
  +   | '(' 'highz1' ',' strength0 ')'
      ;
   
   strength0
  
positive.code:    | attribute_instance* parameter_override
     | attribute_instance* continuous_assign
     | attribute_instance* gate_instantiation
     | attribute_instance* module_instantiation
     | attribute_instance* initial_construct
     | attribute_instance* always_construct
  
negative.code:    : [a-zA-Z_] [a-zA-Z0-9_$]*
     ;
  
  Dollar_Identifier
     : '$' [a-zA-Z0-9_$] [a-zA-Z0-9_$]*
     ;
  
  
  Time_Identifier
     : [0-9] + [mnpf]? 's'
     ;
  
  system_function_identifier
  