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: // | 'swift' '(' '>=' swift_version ')' TODO: Disambiguate with operator
   ;
  
  operating_system : 'macOX' | 'iOS' | 'watchOS' | 'tvOS' ;
  architecture : 'i386' | 'x86_64' | 'arm' | 'arm64' ;
  swift_version : Decimal_literal  '.' Decimal_literal ;
  
  // GRAMMAR OF A LINE CONTROL STATEMENT
  //
  //line_control_statement
  // : '#sourceLocation' '(' 'file:' file_name ',' 'line:' line_number ')' TODO: line could be arg name
  // | '#sourceLocation' '(' ')'
  // ;
  //
  
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 */
  
  