ansi .6.1.2216
<CORE>
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
reference: p4_source in ../src/core.c:1734, export CO SOURCE