add source to instruction

for debug

lib/register/instructions/branch.rb

@@ -3,10 +3,17 @@ module Register
 
   # a branch must branch to a block.
   class Branch < Instruction
-    def initialize to
+    def initialize source , to
+      super(source)
       raise "No block" unless to
       @block = to
     end
-    attr_reader :block
+    attr_reader :block 
+
+    def to_s
+      "Branch(to: #{block.name})"
+    end
+
   end
+
 end