R: split rows into different rows : <p>Say I have a data frame like the following:</p>

<pre><code>&gt; mydf &lt;- data.frame(a=c('A','B','C','D/E','F','G/H','I/J','K','L'), b=c(1,2,3,'4/5',6,'7/8','9/10',11,12))
&gt; mydf
    a    b
1   A    1
2   B    2
3   C    3
4 D/E  4/5
5   F    6
6 G/H  7/8
7 I/J 9/10
8   K   11
9   L   12
</code></pre>

<p>How do I make it look like the following, with an easy one-liner (preferably base)? Thanks</p>

<pre><code>&gt; mydf2
     a    b
1    A    1
2    B    2
3    C    3
4    D    4
5    E    5
6    F    6
7    G    7
8    H    8
9    I    9
10   J   10
11   K   11
12   L   12
</code></pre>
