[求助]feifei公共元素直推法的疑问

Only if A, then
B.         B→A

Must by A, in
order to B.   B→A

If only A, then
B.         A→B

Not A, unless B.
         A→B

A, unless B.             
                    
                        
                            
                                
                                    
                                        
                                            
                                                
                                                    
                                                        
                                                            
                                                                
                                                                    
                                                                        
                                                                        
                                                                        
                                                                            
                                                                            
                                                                                
                                                                                →A,
                    
                    →B

A require B.             A→B

A
depend/draw/base on B.  A→B

这些是从FF笔记里摘出来的，可是在下怎么也搞不明白为啥是这样的？真是知其然不知其所以然啊啊~~

请各位NN从逻辑的角度给点指点好不？谢谢~~~~~~~~~~~~~