anchor.diff:      ;
   
   variable
  -    : (type | 'local') memberName formalParameters*
  +    : (union | 'local') memberName formalParameters*
       ;
   
   // Lexer
  
positive.code:     ;
  
  isCondition
      : 'is' type ( (memberName '=') => memberName specifier | expression )
      -> ^(IS_EXPR type memberName? specifier? expression?)
      ;
  
  controlStructure
  
negative.code:     ;
  
  specialArgument
      : (union | 'local') memberName (containment | specifier)
      //| isCondition
      //| existsCondition
      ;
  