Why Segment Embedding number only 3?


```
import torch.nn as nn
class SegmentEmbedding(nn.Embedding):  
        def __init__(self, embed_size=512):  
                   super().__init__(3, embed_size, padding_idx=0)  
```
This is the source code. First idx is padding, thus only 2 segment is supported. Why does Bert  support 2 segments only?