protected void setup() {
		sync(0);
		p = adjustSeekIndex(0);
	}
Allowed derived classes to modify the behavior of operations which change
the current stream position by adjusting the target token index of a seek
operation. The default implementation simply returns {@code i}. If an
exception is thrown in this method, the current stream index should not be
changed.

<p>For example, {@link CommonTokenStream} overrides this method to ensure that
the seek target is always an on-channel token.</p>

@param i The target token index.
@return The adjusted target token index.
runtime/Java/src/org/antlr/v4/runtime/BufferedTokenStream.java