diff --git a/lib/Language/Elna/Architecture/RiscV.hs b/lib/Language/Elna/Architecture/RiscV.hs
index 6619160..ce64b14 100644
--- a/lib/Language/Elna/Architecture/RiscV.hs
+++ b/lib/Language/Elna/Architecture/RiscV.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Architecture.RiscV
     ( BaseOpcode(..)
     , RelocationType(..)
diff --git a/lib/Language/Elna/Backend/Allocator.hs b/lib/Language/Elna/Backend/Allocator.hs
index 15e8b6f..a56a73b 100644
--- a/lib/Language/Elna/Backend/Allocator.hs
+++ b/lib/Language/Elna/Backend/Allocator.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Backend.Allocator
     ( MachineConfiguration(..)
     , Store(..)
@@ -28,12 +32,19 @@ data Store r
     = RegisterStore r
     | StackStore Int32 r
 
-data AllocationError = AllocationError
+data AllocationError
+    = OutOfRegistersError
+    | UnexpectedProcedureInfoError Info
+    | UndefinedSymbolError Identifier
     deriving Eq
 
 instance Show AllocationError
   where
-    show = const "Ran out of registers during register allocation"
+    show OutOfRegistersError = "Ran out of registers during register allocation"
+    show (UnexpectedProcedureInfoError info) =
+        "Expected to encounter a procedure, got: " <> show info
+    show (UndefinedSymbolError identifier) =
+        concat ["Symbol \"", show identifier, "\" is not defined"]
 
 newtype MachineConfiguration r = MachineConfiguration
     { temporaryRegisters :: [r]
@@ -74,10 +85,14 @@ allocate machineConfiguration globalTable = HashMap.traverseWithKey function
         . runAllocator
         . mapM quadruple
     function :: Identifier -> Vector (Quadruple Variable) -> Either AllocationError (ProcedureQuadruples (Store r))
-    function identifier quadruples' =
-        let Just (ProcedureInfo localTable _) = SymbolTable.lookup identifier globalTable
-            (result, lastState) = run localTable quadruples'
-         in makeResult lastState <$> result
+    function procedureName quadruples' =
+        case SymbolTable.lookup procedureName globalTable of
+            Just (ProcedureInfo localTable _) ->
+                let (result, lastState) = run localTable quadruples'
+                 in makeResult lastState <$> result
+            Just anotherInfo -> Left $ UnexpectedProcedureInfoError anotherInfo
+            Nothing -> Left $ UndefinedSymbolError procedureName
+
     makeResult MachineState{ symbolTable } result = ProcedureQuadruples
         { quadruples = result
         , stackSize = fromIntegral $ SymbolTable.size symbolTable * 4
@@ -155,13 +170,13 @@ operand (VariableOperand variableOperand) =
 storeVariable :: Variable -> Allocator r (Store r)
 storeVariable (TempVariable index) = do
     temporaryRegisters' <- Allocator $ lift $ asks $ getField @"temporaryRegisters"
-    maybe (Allocator $ throwE AllocationError) (pure . RegisterStore)
+    maybe (Allocator $ throwE OutOfRegistersError) (pure . RegisterStore)
         $ temporaryRegisters' !? fromIntegral index
 storeVariable (LocalVariable index) = do
     temporaryRegisters' <- Allocator $ lift $ asks $ getField @"temporaryRegisters"
-    maybe (Allocator $ throwE AllocationError) (pure . StackStore (fromIntegral (succ index) * (-4)))
+    maybe (Allocator $ throwE OutOfRegistersError) (pure . StackStore (fromIntegral (succ index) * (-4)))
         $ temporaryRegisters' !? pred (length temporaryRegisters' - fromIntegral index)
 storeVariable (ParameterVariable index) = do
     temporaryRegisters' <- Allocator $ lift $ asks $ getField @"temporaryRegisters"
-    maybe (Allocator $ throwE AllocationError) (pure . StackStore (fromIntegral index * 4))
+    maybe (Allocator $ throwE OutOfRegistersError) (pure . StackStore (fromIntegral index * 4))
         $ temporaryRegisters' !? fromIntegral index
diff --git a/lib/Language/Elna/Backend/Intermediate.hs b/lib/Language/Elna/Backend/Intermediate.hs
index bb0ae7e..efc1a61 100644
--- a/lib/Language/Elna/Backend/Intermediate.hs
+++ b/lib/Language/Elna/Backend/Intermediate.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Backend.Intermediate
     ( ProcedureQuadruples(..)
     , Operand(..)
diff --git a/lib/Language/Elna/CommandLine.hs b/lib/Language/Elna/CommandLine.hs
index b23be7d..19898e2 100644
--- a/lib/Language/Elna/CommandLine.hs
+++ b/lib/Language/Elna/CommandLine.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.CommandLine
     ( CommandLine(..)
     , commandLine
diff --git a/lib/Language/Elna/Frontend/AST.hs b/lib/Language/Elna/Frontend/AST.hs
index c9e05ef..32ca583 100644
--- a/lib/Language/Elna/Frontend/AST.hs
+++ b/lib/Language/Elna/Frontend/AST.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Frontend.AST
     ( Declaration(..)
     , Identifier(..)
diff --git a/lib/Language/Elna/Frontend/NameAnalysis.hs b/lib/Language/Elna/Frontend/NameAnalysis.hs
index 596ffb2..8e9e7c0 100644
--- a/lib/Language/Elna/Frontend/NameAnalysis.hs
+++ b/lib/Language/Elna/Frontend/NameAnalysis.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Frontend.NameAnalysis
     ( nameAnalysis
     , Error(..)
diff --git a/lib/Language/Elna/Frontend/Parser.hs b/lib/Language/Elna/Frontend/Parser.hs
index 4878b28..1e4499c 100644
--- a/lib/Language/Elna/Frontend/Parser.hs
+++ b/lib/Language/Elna/Frontend/Parser.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Frontend.Parser
     ( Parser
     , programP
diff --git a/lib/Language/Elna/Frontend/SymbolTable.hs b/lib/Language/Elna/Frontend/SymbolTable.hs
index 4333acc..5c2153d 100644
--- a/lib/Language/Elna/Frontend/SymbolTable.hs
+++ b/lib/Language/Elna/Frontend/SymbolTable.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Frontend.SymbolTable
     ( SymbolTable
     , Info(..)
diff --git a/lib/Language/Elna/Frontend/TypeAnalysis.hs b/lib/Language/Elna/Frontend/TypeAnalysis.hs
index 48be782..ea81c4e 100644
--- a/lib/Language/Elna/Frontend/TypeAnalysis.hs
+++ b/lib/Language/Elna/Frontend/TypeAnalysis.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Frontend.TypeAnalysis
     ( typeAnalysis
     , -- Error(..)
diff --git a/lib/Language/Elna/Frontend/Types.hs b/lib/Language/Elna/Frontend/Types.hs
index a3cc730..f7a7e9d 100644
--- a/lib/Language/Elna/Frontend/Types.hs
+++ b/lib/Language/Elna/Frontend/Types.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Frontend.Types
     ( Type(..)
     , addressByteSize
diff --git a/lib/Language/Elna/Glue.hs b/lib/Language/Elna/Glue.hs
index ebb0f69..15089e0 100644
--- a/lib/Language/Elna/Glue.hs
+++ b/lib/Language/Elna/Glue.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Glue
     ( glue
     ) where
diff --git a/lib/Language/Elna/Location.hs b/lib/Language/Elna/Location.hs
index 875aa8d..ccd2c1a 100644
--- a/lib/Language/Elna/Location.hs
+++ b/lib/Language/Elna/Location.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Location
     ( Identifier(..)
     , Location(..)
diff --git a/lib/Language/Elna/Object/Elf.hs b/lib/Language/Elna/Object/Elf.hs
index 8d56dd6..ee1f1af 100644
--- a/lib/Language/Elna/Object/Elf.hs
+++ b/lib/Language/Elna/Object/Elf.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Object.Elf
     ( ByteOrder(..)
     , Elf32_Addr
diff --git a/lib/Language/Elna/Object/ElfCoder.hs b/lib/Language/Elna/Object/ElfCoder.hs
index b01045a..c94292f 100644
--- a/lib/Language/Elna/Object/ElfCoder.hs
+++ b/lib/Language/Elna/Object/ElfCoder.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 -- | Object file generation.
 module Language.Elna.Object.ElfCoder
     ( ElfEnvironment(..)
diff --git a/lib/Language/Elna/Object/StringTable.hs b/lib/Language/Elna/Object/StringTable.hs
index e75f2c6..50193a4 100644
--- a/lib/Language/Elna/Object/StringTable.hs
+++ b/lib/Language/Elna/Object/StringTable.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.Object.StringTable
     ( StringTable
     , append
diff --git a/lib/Language/Elna/RiscV/CodeGenerator.hs b/lib/Language/Elna/RiscV/CodeGenerator.hs
index fffbd39..94e0689 100644
--- a/lib/Language/Elna/RiscV/CodeGenerator.hs
+++ b/lib/Language/Elna/RiscV/CodeGenerator.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.RiscV.CodeGenerator
     ( Directive(..)
     , Statement(..)
diff --git a/lib/Language/Elna/RiscV/ElfWriter.hs b/lib/Language/Elna/RiscV/ElfWriter.hs
index 165197d..d72b5af 100644
--- a/lib/Language/Elna/RiscV/ElfWriter.hs
+++ b/lib/Language/Elna/RiscV/ElfWriter.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 -- | Writer assembler to an object file.
 module Language.Elna.RiscV.ElfWriter
     ( riscv32Elf
diff --git a/rakelib/cross.rake b/rakelib/cross.rake
index 700a8dd..03a69cb 100644
--- a/rakelib/cross.rake
+++ b/rakelib/cross.rake
@@ -1,3 +1,7 @@
+# This Source Code Form is subject to the terms of the Mozilla Public License,
+# v. 2.0. If a copy of the MPL was not distributed with this file, You can
+# obtain one at https://mozilla.org/MPL/2.0/. -}
+
 require 'pathname'
 require 'uri'
 require 'net/http'
diff --git a/rakelib/shared.rb b/rakelib/shared.rb
index 8ecd8cb..d094511 100644
--- a/rakelib/shared.rb
+++ b/rakelib/shared.rb
@@ -1 +1,5 @@
+# This Source Code Form is subject to the terms of the Mozilla Public License,
+# v. 2.0. If a copy of the MPL was not distributed with this file, You can
+# obtain one at https://mozilla.org/MPL/2.0/. -}
+
 TMP = Pathname.new('./build')
diff --git a/rakelib/tester.rake b/rakelib/tester.rake
index d164c2a..4caae20 100644
--- a/rakelib/tester.rake
+++ b/rakelib/tester.rake
@@ -1,3 +1,7 @@
+# This Source Code Form is subject to the terms of the Mozilla Public License,
+# v. 2.0. If a copy of the MPL was not distributed with this file, You can
+# obtain one at https://mozilla.org/MPL/2.0/. -}
+
 require 'open3'
 require 'rake/clean'
 require_relative 'shared'
diff --git a/src/Main.hs b/src/Main.hs
index 164f8e1..aff5360 100644
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Main
     ( main
     ) where
diff --git a/tests/Language/Elna/NameAnalysisSpec.hs b/tests/Language/Elna/NameAnalysisSpec.hs
index 10e6ee0..2fd4c82 100644
--- a/tests/Language/Elna/NameAnalysisSpec.hs
+++ b/tests/Language/Elna/NameAnalysisSpec.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.NameAnalysisSpec
     ( spec
     ) where
diff --git a/tests/Language/Elna/ParserSpec.hs b/tests/Language/Elna/ParserSpec.hs
index 6c91a11..7a142d1 100644
--- a/tests/Language/Elna/ParserSpec.hs
+++ b/tests/Language/Elna/ParserSpec.hs
@@ -1,3 +1,7 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 module Language.Elna.ParserSpec
     ( spec
     ) where
diff --git a/tests/Spec.hs b/tests/Spec.hs
index a824f8c..feacffa 100644
--- a/tests/Spec.hs
+++ b/tests/Spec.hs
@@ -1 +1,5 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 {-# OPTIONS_GHC -F -pgmF hspec-discover #-}