Add hybrid option to selection-declaration.md (#870)

* Add hybrid option to selection-declaration.md

* Update selection-declaration.md

fixed glitch in original edit

* Update selection-declaration.md

* Apply suggestions from code review

Fixing typos

Co-authored-by: Addison Phillips <addison@unicode.org>

* Update selection-declaration.md

* Update exploration/selection-declaration.md

Co-authored-by: Eemeli Aro <eemeli@mozilla.com>

* Update exploration/selection-declaration.md

Co-authored-by: Eemeli Aro <eemeli@mozilla.com>

* Update exploration/selection-declaration.md

Co-authored-by: Eemeli Aro <eemeli@mozilla.com>

---------

Co-authored-by: Addison Phillips <addison@unicode.org>
Co-authored-by: Eemeli Aro <eemeli@mozilla.com>

Author: 3 people

exploration/selection-declaration.md

@@ -356,3 +356,69 @@ and a data model error otherwise.
   Removes some self-documentation from the pattern.
 - Requires the pattern to change if the selectors are modified.
 - Limits number of referenceable selectors to 10 (in the current form)
+
+### Hybrid approach: Match may mutate, no duplicates
+
+In this alternative, in a `.match` statement:
+
+1. variables are mutated by their annotation
+2. no variable can be the operand in two selectors
+
+This keeps most messages more concise, producing the expected results in Example 1.
+
+#### Example 1
+
+```
+.match {$count :integer}
+one {{You have {$count} whole apple.}}
+* {{You have {$count} whole apples.}}
+```
+is precisely equivalent to:
+
+#### Example 2
+```
+.local $count2 = {$count :integer}
+.match {$count2}
+one {{You have {$count2} whole apple.}}
+* {{You have {$count2} whole apples.}}
+```
+
+This avoids the serious problems with mismatched selection and formats
+as in Example 1 under "Do Nothing", whereby the input of `count = 1.2`, 
+results the malformed "You have 1.2 whole apple."
+
+Due to clause 2, this requires users to declare any selector using a `.input` or `.local` declaration
+before writing the `.match`. That is, the following is illegal.
+
+#### Example 3
+```
+.match {$count <anything>}{$count <anything>}
+```
+It would need to be rewritten as something along the lines of:
+
+#### Example 4
+```
+.local $count3 = {$count}
+.match {$count <anything1>}{$count3 <anything2>}
+```
+Notes:
+- The number of times the same variable is used twice in a match (or the older Select) is vanishingly small. Since it is an error — and the advice to fix is easy — that will prevent misbehavior.
+- There would be no change to the ABNF; but there would be an additional constraint in the spec, and relaxation of immutability within the .match statement.
+
+**Pros**
+- No new syntax is required
+- Preserves immutability before and after the .match statement
+- Avoids the serious problem of mismatch of selector and format of option "Do Nothing"
+- Avoids the extra syntax of option "Allow both local and input declarative selectors with immutability"
+- Avoids the problem of multiple variables in "Allow immutable input declarative selectors"
+- Is much more consise than "Match on variables instead of expressions", since it doesn't require a .local or .input for every variable with options
+- Avoids the readability issues with "Provide a #-like Feature"
+
+**Cons**
+- Complexity: `.match` means more than one thing
+- Complexity: `.match` implicitly creates a new lexical scope
+- Violates immutability that we've established everywhere else
+- Requires additional `.local` declarations in cases where a variable would occur twice
+  such as `.match {$date :date option=monthOnly} {$date :date option=full}`
+
+