protected final TypeBearer arg(int n) {
    if (n >= argCount) {
      throw new IllegalArgumentException("n >= argCount");
    }

    try {
      return args[n];
    } catch (ArrayIndexOutOfBoundsException ex) {
      // Translate the exception.
      throw new IllegalArgumentException("n < 0");
    }
  }
Gets the {@code n}th primary argument.

@param n {@code >= 0, < argCount();} which argument
@return {@code non-null;} the indicated argument
android/android_framework/dex-maker/src/main/java/com/android/dx/cf/code/BaseMachine.java