* SOURCE ( -- buffer IN-offset )

the current point of interpret can be gotten through SOURCE.
The buffer may flag out TIB or BLK or a FILE and IN gives
you the offset therein. Traditionally, if the current SOURCE
buffer is used up, REFILL is called that asks for another
input-line or input-block. This scheme would have made it
impossible to stretch an [IF] ... [THEN] over different blocks,
unless [IF] does call REFILL

FORTH/ANS core (dpANS.6.1.2216) ordinary primitive