anchor.diff:      ;
   
   unit // NOTE: Using plurals here because that's the most common case, we could add singulars, but that would allow "within 5 day"
  -    : 'years'
  -    | 'months'
  +    : dateTimePrecision
  +    | pluralDateTimePrecision
  +    | 'week'
       | 'weeks'
  -    | 'days'
  -    | 'hours'
  -    | 'minutes'
  -    | 'seconds'
  -    | 'milliseconds'
       | 'u'STRING // UCUM syntax for units of measure
       ;
   
  
positive.code:     ;
  
  atomicTypeSpecifier
      : IDENTIFIER // TODO: specify atomic type names as part of the grammar?
      ;
  
  listTypeSpecifier
  
negative.code:     ;
  
  sortByItem
      : expressionTerm sortDirection?
      ;
  
  qualifiedIdentifier
  