add WIP section for local var scoping mechanics

Signed-off-by: Alex Suraci <suraci.alex@gmail.com>

029-across-step/proposal.md

@@ -240,6 +240,39 @@ Note: this is the first time a step *modifier* has had additional sibling
 fields. In the event of a field conflict, the `do:` step may be utilized as a
 work-around.
 
+### Var scoping
+
+The inner step will be run with a local var scope that inherits from the outer
+scope and is initialized with the across step's var values. It follows that the
+vars in the local scope are not accessible outside of the `across` step.
+
+Example:
+
+```yaml
+plan:
+- get_var: foo
+  trigger: true
+- run: print
+  type: debug
+  params: {value: ((.:foo))}
+- across:
+  - var: foo
+    values: [one, two]
+  do:
+  - run: print
+    type: debug
+    params: {value: ((.:foo))}
+- run: print
+  type: debug
+  params: {value: ((.:foo))}
+```
+
+Assuming the `get_var` step produces a value of `zero`, this build plan should
+output `zero`, `one`, `two`, `zero`.
+
+TODO: should shadowing vars (as in the above example) be a validation error? or
+maybe a runtime warning? (or allowed?)
+
 
 ## Open Questions