diff --git a/.clang-tidy b/.clang-tidy
new file mode 100644
index 0000000000..e02b1c4556
--- /dev/null
+++ b/.clang-tidy
@@ -0,0 +1 @@
+Checks: '-*,modernize-use-nullptr,modernize-use-override,bugprone,modernize-redundant-void-arg'
diff --git a/libnestutil/config.h.in b/libnestutil/config.h.in
index 8b4d78e471..6ffe368c0d 100644
--- a/libnestutil/config.h.in
+++ b/libnestutil/config.h.in
@@ -129,7 +129,7 @@
 /* "Define if NAN is available" */
 #cmakedefine HAVE_NAN 1
 
-/* "Define if std::nan(NULL) is available" */
+/* "Define if std::nan(nullptr) is available" */
 #cmakedefine HAVE_STD_NAN 1
 
 /* define for ostream */
diff --git a/libnestutil/lockptr.h b/libnestutil/lockptr.h
index 21096cf20a..88c7618864 100644
--- a/libnestutil/lockptr.h
+++ b/libnestutil/lockptr.h
@@ -97,7 +97,7 @@ class lockPTR
     PointerObject( PointerObject const& );
 
   public:
-    explicit PointerObject( D* p = NULL )
+    explicit PointerObject( D* p = nullptr )
       : pointee( p )
       , deletable( true )
       , locked( false )
@@ -114,7 +114,7 @@ class lockPTR
     ~PointerObject()
     {
       assert( not locked );
-      if ( ( pointee != NULL ) and deletable and not locked )
+      if ( pointee and deletable and not locked )
       {
         delete pointee;
       }
@@ -164,34 +164,34 @@ class lockPTR
   // object which must then be initialised, for example
   // by assignement.
 
-  explicit lockPTR( D* p = NULL )
+  explicit lockPTR( D* p = nullptr )
   {
     obj = std::make_shared< PointerObject >( p );
-    assert( obj != NULL );
+    assert( obj );
   }
 
   explicit lockPTR( D& p_o )
   {
     obj = std::make_shared< PointerObject >( p_o );
-    assert( obj != NULL );
+    assert( obj );
   }
 
   lockPTR( const lockPTR< D >& spd )
     : obj( spd.obj )
   {
-    assert( obj != NULL );
+    assert( obj );
   }
 
   virtual ~lockPTR()
   {
-    assert( obj != NULL );
+    assert( obj );
   }
 
   lockPTR< D >
   operator=( const lockPTR< D >& spd )
   {
-    assert( obj != NULL );
-    assert( spd.obj != NULL );
+    assert( obj );
+    assert( spd.obj );
 
     obj = spd.obj;
 
@@ -234,7 +234,7 @@ class lockPTR
   D*
   operator->() const
   {
-    assert( obj->get() != NULL );
+    assert( obj->get() );
 
     return obj->get();
   }
@@ -242,7 +242,7 @@ class lockPTR
   D*
   operator->()
   {
-    assert( obj->get() != NULL );
+    assert( obj->get() );
 
     return obj->get();
   }
@@ -250,7 +250,7 @@ class lockPTR
   D&
   operator*()
   {
-    assert( obj->get() != NULL );
+    assert( obj->get() );
 
     return *( obj->get() );
   }
@@ -258,17 +258,15 @@ class lockPTR
   const D&
   operator*() const
   {
-    assert( obj->get() != NULL );
+    assert( obj->get() );
     return *( obj->get() );
   }
 
 
   bool
-  operator not() const //!< returns true if and only if obj->pointee == NULL
+  operator not() const //!< returns true if and only if not obj->pointee
   {
-    // assert(obj != NULL);
-
-    return ( obj->get() == NULL );
+    return not obj->get();
   }
 
 
@@ -296,51 +294,51 @@ class lockPTR
 
 
   bool
-  valid() const //!< returns true if and only if obj->pointee != NULL
+  valid() const //!< returns true if and only if obj->pointee
   {
-    assert( obj != NULL );
-    return ( obj->get() != NULL );
+    assert( obj );
+    return ( obj->get() );
   }
 
   bool
   islocked() const
   {
-    assert( obj != NULL );
+    assert( obj );
     return ( obj->islocked() );
   }
 
   bool
   deletable() const
   {
-    assert( obj != NULL );
+    assert( obj );
     return ( obj->isdeletable() );
   }
 
   void
   lock() const
   {
-    assert( obj != NULL );
+    assert( obj );
     obj->lock();
   }
 
   void
   unlock() const
   {
-    assert( obj != NULL );
+    assert( obj );
     obj->unlock();
   }
 
   void
   unlock()
   {
-    assert( obj != NULL );
+    assert( obj );
     obj->unlock();
   }
 
   size_t
   references() const
   {
-    return ( obj == NULL ) ? 0 : obj.use_count();
+    return not obj ? 0 : obj.use_count();
   }
 };
 
diff --git a/nest/neststartup.cpp b/nest/neststartup.cpp
index 973da183ff..15d4d992e2 100644
--- a/nest/neststartup.cpp
+++ b/nest/neststartup.cpp
@@ -141,7 +141,7 @@ neststartup( int* argc, char*** argv, SLIInterpreter& engine, std::string module
 #ifdef _IS_PYNEST
   // add the init-script to the list of module initializers
   ArrayDatum* ad = dynamic_cast< ArrayDatum* >( engine.baselookup( engine.commandstring_name ).datum() );
-  assert( ad != NULL );
+  assert( ad );
   ad->push_back( new StringDatum( "(" + modulepath + "/pynest-init.sli) run" ) );
 #endif
 
@@ -160,11 +160,11 @@ nestshutdown( int exitcode )
 Datum*
 CYTHON_unpackConnectionGeneratorDatum( PyObject* obj )
 {
-  Datum* ret = NULL;
-  ConnectionGenerator* cg = NULL;
+  Datum* ret = nullptr;
+  ConnectionGenerator* cg = nullptr;
 
   cg = PNS::unpackConnectionGenerator( obj );
-  if ( cg != NULL )
+  if ( cg )
   {
     ret = static_cast< Datum* >( new ConnectionGeneratorDatum( cg ) );
   }
diff --git a/nest/neststartup.h b/nest/neststartup.h
index 5974e68414..72b790ecd1 100644
--- a/nest/neststartup.h
+++ b/nest/neststartup.h
@@ -33,7 +33,7 @@
 Datum* CYTHON_unpackConnectionGeneratorDatum( PyObject* );
 #else
 #define CYTHON_isConnectionGenerator( x ) 0
-#define CYTHON_unpackConnectionGeneratorDatum( x ) NULL
+#define CYTHON_unpackConnectionGeneratorDatum( x ) nullptr
 #endif
 
 class SLIInterpreter;
diff --git a/nestkernel/conn_builder_conngen.cpp b/nestkernel/conn_builder_conngen.cpp
index b835ebeab9..0f4ccea662 100644
--- a/nestkernel/conn_builder_conngen.cpp
+++ b/nestkernel/conn_builder_conngen.cpp
@@ -82,7 +82,7 @@ ConnectionGeneratorBuilder::connect_()
   if ( num_parameters == 0 )
   {
     // connect source to target
-    while ( cg_->next( source, target, NULL ) )
+    while ( cg_->next( source, target, nullptr ) )
     {
       // No need to check for locality of the target, as the mask
       // created by cg_set_masks() only contains local nodes.
diff --git a/nestkernel/connection_manager.cpp b/nestkernel/connection_manager.cpp
index 0381c80cf4..dd8922dc2e 100644
--- a/nestkernel/connection_manager.cpp
+++ b/nestkernel/connection_manager.cpp
@@ -890,7 +890,7 @@ nest::ConnectionManager::trigger_update_weight( const long vt_id,
   for ( std::vector< ConnectorBase* >::iterator it = connections_[ tid ].begin(); it != connections_[ tid ].end();
         ++it )
   {
-    if ( *it != NULL )
+    if ( *it )
     {
       ( *it )->trigger_update_weight(
         vt_id, tid, dopa_spikes, t_trig, kernel().model_manager.get_connection_models( tid ) );
diff --git a/nestkernel/event.h b/nestkernel/event.h
index f26a6acf17..9d0ae616c6 100644
--- a/nestkernel/event.h
+++ b/nestkernel/event.h
@@ -299,10 +299,10 @@ class Event
    * members, however, in order to avoid the reference of reference
    * problem, we store sender and receiver as pointers and use
    * references in the interface.
-   * Thus, we can still ensure that the pointers are never NULL.
+   * Thus, we can still ensure that the pointers are never nullptr.
    */
-  Node* sender_;   //!< Pointer to sender or NULL.
-  Node* receiver_; //!< Pointer to receiver or NULL.
+  Node* sender_;   //!< Pointer to sender or nullptr.
+  Node* receiver_; //!< Pointer to receiver or nullptr.
 
 
   /**
@@ -620,7 +620,7 @@ class DataLoggingRequest : public Event
   Time recording_offset_;
   /**
    * Names of properties to record from.
-   * @note This pointer shall be NULL unless the event is sent by a connection
+   * @note This pointer shall be nullptr unless the event is sent by a connection
    * routine.
    */
   std::vector< Name > const* const record_from_;
diff --git a/nestkernel/recording_backend_sionlib.cpp b/nestkernel/recording_backend_sionlib.cpp
index 2af52133b7..eb5c7f6d18 100644
--- a/nestkernel/recording_backend_sionlib.cpp
+++ b/nestkernel/recording_backend_sionlib.cpp
@@ -167,7 +167,7 @@ nest::RecordingBackendSIONlib::open_files_()
 
     // we need to delay the throwing of exceptions to the end of the parallel
     // section
-    WrappedThreadException* we = NULL;
+    WrappedThreadException* we = nullptr;
 
     // This code is executed in a parallel region (opened above)!
     const thread t = kernel().vp_manager.get_thread_id();
@@ -228,8 +228,8 @@ nest::RecordingBackendSIONlib::open_files_()
         &sion_chunksize,
         &fs_block_size,
         &rank,
-        NULL,
-        NULL );
+        nullptr,
+        nullptr );
 
       file.buffer.reserve( P_.buffer_size_ );
       file.buffer.clear();
@@ -316,7 +316,7 @@ nest::RecordingBackendSIONlib::close_files_()
     if ( task == 0 )
     {
       int mc;
-      sion_int64* cs = NULL;
+      sion_int64* cs = nullptr;
       int info_blk; // here int, other place sion_int64 due to sion api
       sion_int64 info_pos;
 
@@ -536,14 +536,14 @@ nest::RecordingBackendSIONlib::build_filename_() const
  * ---------------------------------------------------------------- */
 
 nest::RecordingBackendSIONlib::SIONBuffer::SIONBuffer()
-  : buffer_( NULL )
+  : buffer_( nullptr )
   , ptr_( 0 )
   , max_size_( 0 )
 {
 }
 
 nest::RecordingBackendSIONlib::SIONBuffer::SIONBuffer( size_t size )
-  : buffer_( NULL )
+  : buffer_( nullptr )
   , ptr_( 0 )
   , max_size_( 0 )
 {
@@ -552,7 +552,7 @@ nest::RecordingBackendSIONlib::SIONBuffer::SIONBuffer( size_t size )
 
 nest::RecordingBackendSIONlib::SIONBuffer::~SIONBuffer()
 {
-  if ( buffer_ != NULL )
+  if ( buffer_ )
   {
     delete[] buffer_;
   }
@@ -563,7 +563,7 @@ nest::RecordingBackendSIONlib::SIONBuffer::reserve( size_t size )
 {
   char* new_buffer = new char[ size ];
 
-  if ( buffer_ != NULL )
+  if ( buffer_ )
   {
     ptr_ = std::min( ptr_, size );
     memcpy( new_buffer, buffer_, ptr_ );
diff --git a/nestkernel/target_table_devices.h b/nestkernel/target_table_devices.h
index f80d360a35..32855ed47b 100644
--- a/nestkernel/target_table_devices.h
+++ b/nestkernel/target_table_devices.h
@@ -239,7 +239,7 @@ TargetTableDevices::send_from_device( const thread tid,
         it != target_from_devices_[ tid ][ ldid ].end();
         ++it )
   {
-    if ( *it != NULL )
+    if ( *it )
     {
       ( *it )->send_to_all( tid, cm, e );
     }
diff --git a/nestkernel/target_table_devices_impl.h b/nestkernel/target_table_devices_impl.h
index cf29786c9e..095d82c11b 100644
--- a/nestkernel/target_table_devices_impl.h
+++ b/nestkernel/target_table_devices_impl.h
@@ -83,7 +83,7 @@ nest::TargetTableDevices::send_to_device( const thread tid,
         it != target_to_devices_[ tid ][ lid ].end();
         ++it )
   {
-    if ( *it != NULL )
+    if ( *it )
     {
       ( *it )->send_to_all( tid, cm, e );
     }
diff --git a/sli/aggregatedatum.h b/sli/aggregatedatum.h
index c284633f60..517c803061 100644
--- a/sli/aggregatedatum.h
+++ b/sli/aggregatedatum.h
@@ -96,7 +96,7 @@ class AggregateDatum : public TypedDatum< slt >, public C
     // to work.
 
     const AggregateDatum< C, slt >* ddc = dynamic_cast< AggregateDatum< C, slt >* >( const_cast< Datum* >( dat ) );
-    if ( ddc == NULL )
+    if ( not ddc )
     {
       return false;
     }
diff --git a/sli/genericdatum.h b/sli/genericdatum.h
index ea8be03c7e..dc6847d5f3 100644
--- a/sli/genericdatum.h
+++ b/sli/genericdatum.h
@@ -114,7 +114,7 @@ class GenericDatum : public TypedDatum< slt >
 
     //    std::cerr << "d = " << d << " ddc = " << ddc << " dat = " << dat <<
     //    std::endl;
-    if ( ddc == NULL )
+    if ( not ddc )
     {
       return false;
     }
diff --git a/sli/sli_io.cc b/sli/sli_io.cc
index c7bd2c9d70..bc823523b8 100644
--- a/sli/sli_io.cc
+++ b/sli/sli_io.cc
@@ -463,7 +463,7 @@ OstrstreamFunction::execute( SLIInterpreter* i ) const
 #else
   std::ostrstream* out = new std::ostrstream();
 #endif
-  assert( out != NULL );
+  assert( out );
 
   if ( out->good() )
   {
@@ -492,7 +492,7 @@ StrFunction::execute( SLIInterpreter* i ) const
 
   OstreamDatum* ostreamdatum = dynamic_cast< OstreamDatum* >( i->OStack.top().datum() );
 
-  if ( ostreamdatum == NULL )
+  if ( not ostreamdatum )
   {
     OstreamDatum const d;
     Token t = i->OStack.top();
@@ -504,10 +504,10 @@ StrFunction::execute( SLIInterpreter* i ) const
 #else
   std::ostrstream* out = dynamic_cast< std::ostrstream* >( ostreamdatum->get() );
 #endif
-  assert( out != NULL );
+  assert( out );
   ostreamdatum->unlock();
 
-  if ( out != NULL )
+  if ( out )
   {
     if ( out->good() )
     {
diff --git a/sli/sliactions.cc b/sli/sliactions.cc
index 756ea1506e..3e32e572e4 100644
--- a/sli/sliactions.cc
+++ b/sli/sliactions.cc
@@ -106,7 +106,6 @@ FunctiontypeFunction::execute( SLIInterpreter* i ) const
 void
 TrietypeFunction::execute( SLIInterpreter* i ) const
 {
-
   TrieDatum* tried = static_cast< TrieDatum* >( i->EStack.top().datum() );
   i->EStack.top().assign_by_ref( tried->lookup( i->OStack ) );
 }
@@ -114,17 +113,14 @@ TrietypeFunction::execute( SLIInterpreter* i ) const
 void
 CallbacktypeFunction::execute( SLIInterpreter* i ) const
 {
-  //    assert(i->ct.datum() != NULL); // we wouldn't be here otherwise
-
   CallbackDatum* cb = static_cast< CallbackDatum* >( i->ct.datum() );
 
   // Note, although cb is a pointer to a class derived from Datum,
   // it also has the properties of a token, since it is derived from both.
 
-
   i->EStack.push_move( i->ct );
   // This moves the complete callback datum to the EStack.
-  // Now, the pointer in ct is set to NULL !!
+  // Now, the pointer in ct is set to nullptr !!
 
   // Now push command to restore the callback, once the action has
   // been finished
diff --git a/sli/sliarray.cc b/sli/sliarray.cc
index 64cf2dd332..6561321c52 100644
--- a/sli/sliarray.cc
+++ b/sli/sliarray.cc
@@ -1610,26 +1610,21 @@ void
 SLIArrayModule::IMapThreadFunction::execute( SLIInterpreter* i ) const
 {
   ProcedureDatum* procd = static_cast< ProcedureDatum* >( i->EStack.pick( 1 ).datum() );
-  //    assert(procd != NULL);
 
   size_t proclimit = procd->size();
 
   IntegerDatum* argcountd = static_cast< IntegerDatum* >( i->EStack.pick( 2 ).datum() );
-  //  assert(argcountd != NULL);
 
   size_t argcount = argcountd->get();
 
   IntegerDatum* proccountd = static_cast< IntegerDatum* >( i->EStack.pick( 3 ).datum() );
-  // assert(proccountd != NULL);
+
   size_t proccount = proccountd->get();
 
   ArrayDatum* sarray = static_cast< ArrayDatum* >( i->EStack.pick( 4 ).datum() );
-  // assert(sarray != NULL);
   ArrayDatum* tarray = static_cast< ArrayDatum* >( i->EStack.pick( 5 ).datum() );
-  // assert(tarray != NULL);
 
   IntegerDatum* limitd = static_cast< IntegerDatum* >( i->EStack.pick( 6 ).datum() );
-  // assert(limitd != NULL);
 
   size_t args = sarray->size(); // number of argument arrays
   size_t limit = limitd->get(); // number of arguments per array
@@ -2070,7 +2065,7 @@ SLIArrayModule::AreaFunction::execute( SLIInterpreter* i ) const
   IntegerDatum* a_y_d = dynamic_cast< IntegerDatum* >( i->OStack.pick( 1 ).datum() );
   IntegerDatum* a_x_d = dynamic_cast< IntegerDatum* >( i->OStack.pick( 0 ).datum() );
 
-  //   if(s_h_d == NULL)
+  //   if(not s_h_d)
   //   {
   //     i->raiseerror(i->ArgumentTypeError);
   //     return;
@@ -2334,12 +2329,12 @@ SLIArrayModule::Area2Function::execute( SLIInterpreter* i ) const
   IntegerDatum* a_y_d = dynamic_cast< IntegerDatum* >( i->OStack.pick( 1 ).datum() );
   IntegerDatum* a_x_d = dynamic_cast< IntegerDatum* >( i->OStack.pick( 0 ).datum() );
 
-  //   if(s_h_d == NULL)
+  //   if(not s_h_d)
   //   {
   //     i->raiseerror(i->ArgumentTypeError);
   //     return;
   //   }
-  //   if(s_w_d == NULL)
+  //   if(not s_w_d)
   //   {
   //     i->raiseerror(i->ArgumentTypeError);
   //     return;
diff --git a/sli/slitype.h b/sli/slitype.h
index b9a3523543..4b38e74199 100644
--- a/sli/slitype.h
+++ b/sli/slitype.h
@@ -86,8 +86,6 @@ class SLIType
   SLIFunction*
   getaction() const
   {
-    // assert(defaultaction != NULL);
-    // If this fails, we have created a datum before the types were initialised.
     return defaultaction;
   }
 };
diff --git a/sli/specialfunctionsmodule.cc b/sli/specialfunctionsmodule.cc
index b3a850629b..e3c495bc5f 100644
--- a/sli/specialfunctionsmodule.cc
+++ b/sli/specialfunctionsmodule.cc
@@ -444,11 +444,11 @@ SpecialFunctionsModule::ErfcFunction::execute( SLIInterpreter* i ) const
 
 // ---------------------------------------------------------------
 
-SpecialFunctionsModule::GaussDiskConvFunction::GaussDiskConvFunction( void )
+SpecialFunctionsModule::GaussDiskConvFunction::GaussDiskConvFunction()
 {
 }
 
-SpecialFunctionsModule::GaussDiskConvFunction::~GaussDiskConvFunction( void )
+SpecialFunctionsModule::GaussDiskConvFunction::~GaussDiskConvFunction()
 {
 }
 
diff --git a/sli/tarrayobj.cc b/sli/tarrayobj.cc
index ded44e05cf..2a8e978386 100644
--- a/sli/tarrayobj.cc
+++ b/sli/tarrayobj.cc
@@ -89,7 +89,7 @@ TokenArrayObj::allocate( size_t new_s, size_t new_c, size_t new_a, const Token&
   assert( new_a != 0 );
 
   Token* h = new Token[ new_c ];
-  assert( h != NULL );
+  assert( h );
 
   if ( t != Token() )
   {
@@ -123,7 +123,7 @@ TokenArrayObj::allocate( size_t new_s, size_t new_c, size_t new_a, const Token&
     delete[] p;
   }
   p = h;
-  assert( p != NULL );
+  assert( p );
 
   ++allocations;
 }
@@ -278,7 +278,7 @@ TokenArrayObj::erase( Token* first, Token* last )
   {
     if ( to->p )
     {
-      // deleting NULL pointer is safe in ISO C++
+      // deleting nullptr pointer is safe in ISO C++
       to->p->removeReference();
     }
     to->p = from->p;   // move
@@ -399,7 +399,7 @@ TokenArrayObj::insert( size_t i, size_t n, const Token& t )
     to->p = from->p;   // move
     from->p = nullptr; // knowing that to->p is
     --from;
-    --to; // NULL before
+    --to; // nullptr before
   }
 
   for ( size_t i = 0; i < n; ++i ) // insert n copies of Token t;
@@ -426,7 +426,7 @@ TokenArrayObj::insert_move( size_t i, TokenArrayObj& a )
     to->p = from->p;   // move
     from->p = nullptr; // knowing that to->p is
     --from;
-    --to; // NULL before
+    --to; // nullptr before
   }
 
   from = a.p;
@@ -498,7 +498,7 @@ TokenArrayObj::insert_move( size_t i, Token& t )
     to->p = from->p;   // move
     from->p = nullptr; // knowing that to->p is
     --from;
-    --to; // NULL before
+    --to; // nullptr before
   }
 
   ( p + i )->p = t.p; // move contens of t
@@ -549,7 +549,7 @@ TokenArrayObj::replace_move( size_t i, size_t n, TokenArrayObj& a )
     {
       if ( to->p )
       {
-        // deleting NULL pointer is safe in ISO C++
+        // deleting nullptr pointer is safe in ISO C++
         to->p->removeReference();
       }
       to->p = from->p;   // move
@@ -586,7 +586,7 @@ TokenArrayObj::replace_move( size_t i, size_t n, TokenArrayObj& a )
       to->p->removeReference();
     }
     to->p = from->p;   // movement, it is typically
-    from->p = nullptr; // not the NULL pointer
+    from->p = nullptr; // not the nullptr pointer
     ++from;
     ++to;
   }
@@ -603,7 +603,7 @@ TokenArrayObj::append_move( TokenArrayObj& a )
 
   while ( from < a.end() ) // move
   {                        // knowing that to->p is
-    to->p = from->p;       // NULL before
+    to->p = from->p;       // nullptr before
     from->p = nullptr;
     ++from;
     ++to;
diff --git a/sli/token.h b/sli/token.h
index acb07077d8..e5a7be51e2 100644
--- a/sli/token.h
+++ b/sli/token.h
@@ -373,7 +373,6 @@ class Token
   Datum*
   operator->() const
   {
-    //      assert(p!= NULL);
     return p;
   }
 
@@ -381,7 +380,6 @@ class Token
   Datum&
   operator*() const
   {
-    //      assert(p != NULL);
     return *p;
   }
 
diff --git a/sli/tokenutils.h b/sli/tokenutils.h
index 2523feaae3..64c92195d5 100644
--- a/sli/tokenutils.h
+++ b/sli/tokenutils.h
@@ -152,7 +152,7 @@ FT
 getValue( const Token& t )
 {
   FT* value = dynamic_cast< FT* >( t.datum() );
-  if ( value == NULL )
+  if ( not value )
   {
     throw TypeMismatch();
   }
@@ -177,7 +177,7 @@ void
 setValue( const Token& t, FT const& value )
 {
   FT* old = dynamic_cast< FT* >( t.datum() );
-  if ( old == NULL )
+  if ( not old )
   {
     throw TypeMismatch();
   }
diff --git a/sli/typechk.cc b/sli/typechk.cc
index 50e9e8c400..92e055dfab 100644
--- a/sli/typechk.cc
+++ b/sli/typechk.cc
@@ -167,7 +167,6 @@ TypeTrie::getalternative( TypeTrie::TypeNode* pos, const Name& type )
   if ( pos->type == empty )
   {
     pos->type = type;
-    //    assert(pos->next == NULL);
     return pos;
   }
 
@@ -180,10 +179,8 @@ TypeTrie::getalternative( TypeTrie::TypeNode* pos, const Name& type )
 
     if ( pos->type == sli::any )
     {
-      // any must have been the tail and the previous
-      // if must have added an extra Node, thus the following
-      // assertion must hold:
-      // assert(pos->alt->alt == NULL);
+      // When entering this if, the previous if added an extra Node,
+      // thus pos->alt->alt will not be defined yet (nullptr)
 
       TypeNode* new_tail = pos->alt;
 
@@ -253,9 +250,9 @@ TypeTrie::insert_move( const TypeArray& a, Token& f )
   }
 
   /* Error conditions:
-     1. If pos->next!=NULL, the parameter list overlaps with
+     1. If pos->next != nullptr, the parameter list overlaps with
      an existing function definition.
-     2. If pos->alt != NULL, something undefined must have happened.
+     2. If pos->alt != nullptr, something undefined must have happened.
      This should be impossible.
   */
   if ( not pos->next )