Hi I want to delete the rows with the entries whose number of occurrence is smaller than a number, for example:

```
df = pd.DataFrame({'a': [1,2,3,2], 'b':[4,5,6,7], 'c':[0,1,3,2]})
df

```

```none
   a  b  c
0  1  4  0
1  2  5  1
2  3  6  3
3  2  7  2

```

Here I want to delete all the rows if the number of occurrence in column 'a' is less than twice.  

Wanted output:

```none
   a  b  c
1  2  5  1
3  2  7  2

```

What I know: 
we can find the number of occurrence by `condition = df['a'].value_counts() < 2`, and it will give me something like:

```none
2    False
3    True
1    True
Name: a, dtype: int64

```

But I don't know how I should approach from here to delete the rows.  

Thanks in advance!