anchor.diff:  
   // GRAMMAR OF A BUILD CONFIGURATION STATEMENT
   
  -conditional_compilation_block : if_directive_clause elseif_directive_clauses? else_directive_clause ;
  +conditional_compilation_block : if_directive_clause elseif_directive_clauses? else_directive_clause? endif_directive ;
   if_directive_clause : if_directive compilation_condition statements? ;
   elseif_directive_clauses : elseif_directive_clause+ ;
   elseif_directive_clause : elseif_directive compilation_condition statements? ;
  
positive.code: Platform_name
   : 'iOS' | 'iOSApplicationExtension'
   | 'macOS' | 'macOSApplicationExtension'
   | 'watchOS'
   | 'tvOS'
   ;
  
negative.code:  characters are not treated as a single token that may then be
   misinterpreted as a bit shift >> operator.
  */
  
  /* these following tokens are also a Binary_operator so much come first as special case */
  
  