There are two expressions in HCL2 that are used to iterate over
collections:
- Splat expressions, e.g. `foo.*.bar`, and
- For expressions, e.g. `[for v in foo: v.bar]`
In both of these cases, the parts of the expression that are not the
collection being iterated behave like callbacks, and must be treated as
such by the apply rewriter.
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