Prevent property duplication within a ruleset. (#7)

Author: nsaunders

CHANGELOG.md

@@ -7,6 +7,8 @@ Notable changes are documented in this file. The format is based on [Keep a Chan
 Breaking changes:
 
 New features:
+- A single property appearing more than once within a ruleset now results in a
+  compiler error.
 
 Bugfixes:
 

examples/TypeSafety.purs

@@ -13,6 +13,16 @@ import Tecton.Rule as Rule
 main :: Effect Unit
 main = log $ renderSheet pretty do
 
+  {-
+  -- Error: Property appears twice within a single ruleset.
+  universal ? Rule.do
+    color := white
+    color := black
+  -}
+
+  -- Works
+  universal ? color := white
+
   -- Animations
 
   let fade = keyframesName "fade"

examples/output/TypeSafety.css

@@ -1,3 +1,6 @@
+* {
+  color: hsl(0.0, 0.0%, 100.0%);
+}
 @keyframes fade {
   0% {
     opacity: 0;