Dynamic LOOP AT ... WHERE (cond) only handled IN in a narrow lower-case form. Upper case conditions silently matched nothing (or crashed), and NOT IN produced invalid JavaScript.

DATA range TYPE RANGE OF i.
DATA dynamic TYPE string.

dynamic = 'FIELD IN RANGE'.      " -> TypeError, operand resolved to undefined
dynamic = 'field not in range'.  " -> SyntaxError: Unexpected identifier 'range'

LOOP AT tab INTO row WHERE (dynamic).
ENDLOOP.

Changes in packages/runtime/src/statements/loop.ts (dynamicToWhere):

• Case insensitive operands — the right hand operand is lower cased before evaluate(...), matching the lower case identifiers emitted by the transpiler. ABAP conditions are typically written in upper case.
• NOT IN — comparator regex now matches NOT IN as one token and emits !abap.compare.in(...). Previously it parsed as comparator not with operand in, leaving a dangling identifier in the evaluated expression.
• Field symbol operand — FIELD IN <RANGE> resolves to evaluate("fs_range_").getPointer(), so the range table behind the field symbol is passed to abap.compare.in.

Tests added in test/statements/loop.ts covering upper case IN, NOT IN, and a field symbol range.

• Fixes dynamic loop where IN #1596