Off topic:[SOLVED] How to expand a data.frame?

Question

[catechize.2.learn]

I have a dataframe like:

    a= data.frame (A= c('a','b;c;d'),B= c('e','f'))
a    
      A B
1     a e
2 b;c;d f

How to convert it (efficiently) to a dataframe of

       A B
 1     a e
 2     b f
 3     c f
 4     d f

Thank you.