First pass at a grammar.

grammar.md

+Initial pass at a Sass grammar. Nowhere close to complete.
+              
+    stylesheet    ->  statement*
+              
+    statement     ->  ruleset
+              
+    ruleset       ->  SELECTOR '{' declarations '}'
+              
+    declarations  ->  rule [';' declarations]?
+                  ->  ruleset declarations?
+                  ->  []
+              
+    rule          ->  property ':' value
+                  ->  []
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