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