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