neither... nor/RelEx

Example : "I could use neither the lorry nor the van. " 
 		 	 _to-do(could, use) 
    			_quantity(lorry, neither) 
    			_det(lorry, neither) 
  		 	conj_nor(lorry, van) 
    			_obj(use, lorry) 
    			_obj(use, van) 
   			 _subj(use, I) 
yes ,it doesn't have nothing to do with conjunction 
But, when I test conjunction I notice  
The above rule generate
"Summer school is not easy but difficult."
_advmod(be, not) 

@ruiting suggest me to represent it 
NEGATIVE-FLAG(easy, T)
also 
for "Pizza, which most people love, is not very healthy."
_advmod(be, not) 
is not correct ,right?