anchor.diff:     ;
   
   factor
  -   : 
  +   :
      vara
      | number
      ;
  
positive.code:    ;
  
  expression
     : ('+' | '-' | 'ε')? term (('+' | '-') term)*
     ;
  
  term
  
negative.code:    ;
  
  relop
     : ('<' ('>' | '=' )?)
     | ('>' ('<' | '=' )?)
     | '='
     | '+'
     | '-'
  