Cleaning up whitespace spec

Add requirements, clarify technical choices, add examples.

exploration/pattern-exterior-whitespace.md

@@ -58,6 +58,9 @@ _What use-cases do we see? Ideally, quote concrete examples._
 
 _What properties does the solution have to manifest to enable the use-cases above?_
 
+- It **_MUST_** be possible to include _message_-meaningful whitespace at the start or end of
+  a _pattern_.
+
 ## Constraints
 
 _What prior decisions and existing conditions limit the possible design?_
@@ -70,13 +73,27 @@ _What prior decisions and existing conditions limit the possible design?_
 
 _Describe the proposed solution. Consider syntax, formatting, errors, registry, tooling, interchange._
 
-- Patterns MAY be quoted using solution 4
-- User can quote whitespace using solutions 1 and 2
+- All unquoted whitespace on the exterior of a pattern is not meaningful and is removed
+  during formatting.
+- Patterns MAY be quoted (solution 4). Whitespace inside the quoting syntax is meaningful.
+  >```
+  >when foo bar {{   This pattern has exactly two spaces at the start and two at the end  }}  when * *
+  >```
+- A pattern can begin or end with a quoted literal containing whitespace (or any other allowed characters).
+  All whitespace within a pattern is meaningful.
+  The quoted literal may be empty.
+  >```
+  >when foo bar  {|  |}  This pattern has four spaces at the start and two at the end{|  |} when * *
+  >when foo bar  {||}    This pattern also has four spaces at the start and two at the end  {||} when * *
+  >```
 
 Still being discussed:
 
 - Whether unquoted "simple" patterns are trimmed.
   This will be dealt with in the syntax spec.
+  >```
+  >    Does this message have spaces at the start/end or not? {$user} wants to know.  
+  >```
 
 ## Alternatives Considered