Exercise 2: Why WS-CURRENT-STEP Genuinely Needs to Exist for the EVALUATE Rewrite — Possible Solution
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WHAT THE ORIGINAL ALTER VERSION'S OWN REAL STATE ACTUALLY WAS
In this chapter's own original ALTER example, the "memory" of which
step should run next wasn't stored in any real data field at all - it
was stored implicitly, inside PROCESS-PARA's own GO TO statement
itself, in the form of whatever paragraph name that GO TO currently
pointed to. ALTER's whole real mechanism works by rewriting that
target destination directly, using the GO TO statement's own real
target as the place where "which step comes next" is remembered.

WHY EVALUATE HAS NO EQUIVALENT PLACE TO STORE THAT SAME INFORMATION
EVALUATE, unlike GO TO, doesn't have a real, rewritable "target" the
way a GO TO statement does - it's a real, structured statement that
picks one branch to execute based on comparing a real data value
against a set of WHEN conditions. There's no real mechanism resembling
ALTER available to EVALUATE at all; it has nothing analogous to a GO
TO's own destination that could be silently modified from elsewhere.

WHY A REAL, SEPARATE FIELD IS GENUINELY NECESSARY INSTEAD
Since EVALUATE decides its own real branch by testing a value, and
the original version's own "next step" information can no longer live
inside a rewritable GO TO target, that same information has to be
moved somewhere EVALUATE can genuinely test it - a real, ordinary
WORKING-STORAGE field. This chapter's own rewrite introduces
WS-CURRENT-STEP specifically to hold that value explicitly, and moves
"B" into it at the exact point the original version's own ALTER
statement used to silently rewrite the GO TO target.

WHY THIS GENUINELY REPRODUCES THE ORIGINAL BEHAVIOR
By storing "A" as the field's own real starting value and updating it
to "B" once STEP A's processing completes, WS-CURRENT-STEP genuinely
recreates the same real progression the ALTER version produced -
STEP A processing runs first, then STEP B processing runs on the next
execution - but now through a value EVALUATE can directly and visibly
test, rather than through an invisible, rewritten GO TO destination.

ANSWER: WS-CURRENT-STEP needs to exist as a real, separate field
because the original ALTER version stored "which step runs next"
implicitly inside PROCESS-PARA's own GO TO target, and EVALUATE has no
equivalent rewritable destination it could use the same way - it can
only test a real, explicit data value. Introducing WS-CURRENT-STEP
gives EVALUATE something concrete to test, and moving "B" into it
where the original ALTER statement used to run reproduces the same
real progression from step A to step B, just through a visible field
instead of a silently modified GO TO target.

WHY THIS WORKS AS AN ANSWER
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This correctly identifies that ALTER's own real mechanism stored state
implicitly in a GO TO's target, while EVALUATE requires that same
state to be made explicit in a real data field, and explains why the
rewrite's own placement of the field update reproduces the original
version's behavior.