public byte[] getPayload(byte[] data,
    int offset) throws IOException
Returns the payload data at the current term position.
This is invalid until #nextPosition()  is called for
the first time.
This method must not be called more than once after each call
of #nextPosition() . However, payloads are loaded lazily,
so if the payload data for the current position is not needed,
this method may not be called at all for performance reasons.

 public int getPayloadLength()
Returns the length of the payload at the current term position.
This is invalid until #nextPosition()  is called for
the first time.

 public boolean isPayloadAvailable()
Checks if a payload can be loaded at this position.

Payloads can only be loaded once per call to
#nextPosition() .

 public int nextPosition() throws IOException
Returns next position in the current document. It is an error to call
this more than #freq()  times
without calling #next() 
 This is
invalid until #next()  is called for
the first time.