(self, p)
feature : WORD WORD
                | feature WORD
feature : WORD WORD
                | feature WORD
feature : WORD WORD | feature WORD
def p_feature_words(self, p):
        '''feature : WORD WORD
                | feature WORD'''
        p[0] = ' '.join([p[1], p[2]])
def p_feature_words ( self , p ) : p [ 0 ] = ' ' . join ( [ p [ 1 ] , p [ 2 ] ] )