anchor.diff:     ;
   
   relop
  -   : '<' ('>' | '=' | 'ε')
  -   | '>' ('<' | '=' | 'ε')
  +   : '<' ('>' | '=' | 'ε')?
  +   | '>' ('<' | '=' | 'ε')?
      | '='
  +   | '+'
  +   | '-'
      ;
   
   
  
positive.code:    ;
  
  expression
     : ('+' | '-' | 'ε') term (('+' | '-') term)*
     ;
  
  term
  
negative.code: 
  statement
     : 'PRINT' exprlist
     | 'IF' expression relop expression 'THEN'? statement
     | 'GOTO' number
     | 'INPUT' varlist
     | 'LET'? vara '=' expression
     | 'GOSUB' expression
     | 'RETURN'
     | 'CLEAR'
  