    %token  ID 
    %left  '-'
    %left  '*'
    %right UNARY
    
    %%
    
    expr:
        expr '-' term
    | 
        term
    ;
    
    term:
        term '*' factor
    | 
        factor
    ;
    
    factor:
       '-' expr %prec UNARY
    | 
        ID
    ;
