BUGFIX: Register/unregister variables added/remove from existing assi…

…gnment statement.

src/Core/Echo.Ast/AssignmentStatement.cs

@@ -1,5 +1,4 @@
 using System.Collections.Generic;
-using System.Linq;
 using Echo.Code;
 using Echo.Graphing;
 
@@ -30,7 +29,9 @@ public AssignmentStatement(IVariable variable, Expression<TInstruction> expressi
         public AssignmentStatement(IEnumerable<IVariable> variables, Expression<TInstruction> expression)
         {
             Expression = expression;
-            Variables = variables.ToList();
+            Variables = new VariableCollection<TInstruction>(this);
+            foreach (var variable in variables)
+                Variables.Add(variable);
             OriginalRange = expression.OriginalRange;
         }
 

src/Core/Echo.Ast/AstNode.cs

@@ -1,5 +1,9 @@
+using System.Collections.Generic;
+using System.IO;
 using Echo.Code;
+using Echo.ControlFlow.Serialization.Dot;
 using Echo.Graphing;
+using Echo.Graphing.Serialization.Dot;
 
 namespace Echo.Ast
 {

src/Core/Echo.Ast/VariableCollection.cs

@@ -0,0 +1,46 @@
+using System.Collections.ObjectModel;
+using Echo.Code;
+
+namespace Echo.Ast;
+
+internal sealed class VariableCollection<TInstruction> : Collection<IVariable>
+{
+    private readonly Statement<TInstruction> _owner;
+
+    public VariableCollection(Statement<TInstruction> owner)
+    {
+        _owner = owner;
+    }
+
+    protected override void InsertItem(int index, IVariable item)
+    {
+        base.InsertItem(index, item);
+        _owner.GetParentCompilationUnit()?.RegisterVariableWrite(item, _owner);
+    }
+
+    protected override void SetItem(int index, IVariable item)
+    {
+        var root = _owner.GetParentCompilationUnit();
+        root?.UnregisterVariableWrite(Items[index], _owner);
+        base.SetItem(index, item);
+        root?.RegisterVariableWrite(item, _owner);
+    }
+
+    protected override void RemoveItem(int index)
+    {
+        var variable = Items[index];
+        base.RemoveItem(index);
+        _owner.GetParentCompilationUnit()?.UnregisterVariableWrite(variable, _owner);
+    }
+
+    protected override void ClearItems()
+    {
+        if (_owner.GetParentCompilationUnit() is {} root)
+        {
+            foreach (var item in Items)
+                root.UnregisterVariableWrite(item, _owner);
+        }
+
+        base.ClearItems();
+    }
+}