AT&T 6300 Programmer's Manual page 267

FOR...NEXT
Statements
Nested Loops
FOR ... NEXT loops may be nested. A nested
loop may be placed within the context of
another FOR ... NEXT loop. When loops are
nested, each loop must have a unique variable
name as its counter. The NEXT statement for
the inside loop must appear before that for the
outside loop. If nested loops have the same
end point, a single NEXT statement may be
used for all of them. A statement of this form:
NEXT V1, V2, V3
performs the same action as this sequence of
statements:
NEXT V1
NEXT V2
NEXT V3
The variable(s) in the NEXT statement may
be omitted, in which case the NEXT statement
matches the most recent FOR statement.
If a NEXT statement is encountered before its
corresponding FOR statement, a "NEXT
without FOR" error message is issued and
execution is terminated.
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