anchor.diff:      ;
       
   resource
  -    : (variableStart) => variable specifier
  +    : controlVariableOrExpression
  +    ;
  +
  +controlVariableOrExpression
  +    : (variableStart) => variable specifier 
       | expression
       ;
   
  
positive.code:     :   '->'
      ;
  
  RANGE
      :   '..'
      ;
      
  COMPARE
      :   '<=>'
      ;
  
negative.code: // Control structures.
  
  condition
      : expression | existsCondition | nonemptyCondition | isCondition
      ;
  
  existsCondition
      : 'exists' controlVariableOrExpression
      ;
      
  nonemptyCondition
      : 'nonempty' controlVariableOrExpression
      ;
      
  isCondition
  