[DESIGN] Effect of selectors on placeholders

Addresses #736, #747 

DO NOT REVIEW YET == WORK IN PROGRESS

Author: aphillips

exploration/selection-declaration.md

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+# Effect of Selectors on Subsequent Placeholders
+
+Status: **Proposed**
+
+<details>
+	<summary>Metadata</summary>
+	<dl>
+		<dt>Contributors</dt>
+		<dd>@aphillips</dd>
+		<dt>First proposed</dt>
+		<dd>2024-03-27</dd>
+		<dt>Pull Requests</dt>
+		<dd>#000</dd>
+	</dl>
+</details>
+
+## Objective
+
+_What is this proposal trying to achieve?_
+
+Define what effect (if any) the _annotation_ of a _selector_ has on subsequent _placeholders_
+that access the same _operand_ value.
+
+## Background
+
+_What context is helpful to understand this proposal?_
+
+In MFv2, we require that all _selectors_ have an _annotation_.
+
+Ideally, a _selector_ and a _formatter_ for the same _function_ operating on the same _operand_ 
+use the same options.
+
+```
+.input {$num :number minimumFractionDigits=2}
+.match {$num}
+one {{Unreachable in English locale}}
+*   {{This prints {$num} with two decimal places.}}
+```
+
+It is tempting to want to write this as a shorthand:
+
+```
+.match {$num :number minimumFractionDigits=2}
+one {{Unreachable...}}
+*   {{This prints {$num} with two decimal places.}}
+```
+
+## Use-Cases
+
+_What use-cases do we see? Ideally, quote concrete examples._
+
+**Multiple Selection Conflicts**
+If a user writes multiple selectors for the same operand, which one formats the placeholder?
+
+```
+.match {$num :integer} {$num :number minimumFractionDigits=2}
+* * {{What is printed for {$num}?}}
+
+.match {$num :number minimumFractionDigits=2} {$num :integer}
+* * {{What is printed for {$num}?}}
+```
+
+## Requirements
+
+_What properties does the solution have to manifest to enable the use-cases above?_
+
+## Constraints
+
+_What prior decisions and existing conditions limit the possible design?_
+
+## Proposed Design
+
+_Describe the proposed solution. Consider syntax, formatting, errors, registry, tooling, interchange._
+
+## Alternatives Considered
+
+_What other solutions are available?_
+_How do they compare against the requirements?_
+_What other properties they have?_