anchor.diff:  pbx_build_file
       : REFERENCE '=' '{'
           isa_pbx_build_file
  -        file_ref
  +        file_ref?
  +        platform_filter?
  +        product_ref?
           settings?
         '}' ';'
       ;
  
positive.code: 
  file_ref
      : 'fileRef' '=' REFERENCE ';'
      | 'productRef' '=' REFERENCE ';'
      ;
  
  container_portal
  
negative.code: 
  uses_tabs
      : 'usesTabs' '=' NUMBER ';'
      ;
  
  platform_filter
      : 'platformFilter' '=' (QUOTED_STRING|NON_QUOTED_STRING) ';'
      ;
  
  children
  