anchor.diff:   characters are not treated as a single token that may then be
    misinterpreted as a bit shift >> operator.
   */
  -
  -//operator : Binary_operator | Prefix_operator | Postfix_operator ;
   
   /* these following tokens are also a Binary_operator so much come first as special case */
   
  
positive.code: 
  // GRAMMAR OF A BUILD CONFIGURATION STATEMENT
  
  conditional_compilation_block : if_directive_clause elseif_directive_clauses? else_directive_clause ;
  if_directive_clause : if_directive compilation_condition statements? ;
  elseif_directive_clauses : elseif_directive_clause+ ;
  elseif_directive_clause : elseif_directive compilation_condition statements? ;
  
negative.code: Platform_name
   : 'iOS' | 'iOSApplicationExtension'
   | 'macOS' | 'macOSApplicationExtension'
   | 'OSX' | 'OSXApplicationExtension' // still allowed in swift 3.0
   | 'watchOS'
   | 'tvOS'
   ;
  