I found the following piece of code:

```
/* WMF or IF2 frames */
if ((input_format == MIME_IETF) | (input_format == IF2)) {
  ....

```

Source: <https://android.googlesource.com/platform/frameworks/av/+/android-7.1.2_r11/media/libstagefright/codecs/amrnb/dec/src/amrdecode.cpp#414>

I don't believe this is the programmer's real intention. But I found it can't cause real problem. I want to know if this will cause problem.