diff --git a/src/digraphs-debug.h b/src/digraphs-debug.h
index 8f660e574..d144cc2b2 100644
--- a/src/digraphs-debug.h
+++ b/src/digraphs-debug.h
@@ -21,10 +21,11 @@
 #ifndef DIGRAPHS_SRC_DIGRAPHS_DEBUG_H_
 #define DIGRAPHS_SRC_DIGRAPHS_DEBUG_H_
 
-#include <assert.h>
-#include <src/system.h>
+// C headers
+#include <assert.h>  // for assert
 
-#include "digraphs-config.h"
+// Digraphs package headers
+#include "digraphs-config.h"  // for DIGRAPHS_KERNEL_DEBUG
 
 // DIGRAPHS_ASSERT is a version of 'assert' which is enabled by the
 // configure option --enable-debug
diff --git a/src/digraphs.c b/src/digraphs.c
index 6d1d7d544..f7a4b046e 100644
--- a/src/digraphs.c
+++ b/src/digraphs.c
@@ -12,7 +12,16 @@
 **
 *******************************************************************************/
 
-#include "src/digraphs.h"
+#include "digraphs.h"
+
+#include <stdbool.h>
+#include <stdlib.h>
+
+#include "bliss-0.73/bliss_C.h"
+
+#include "digraphs-debug.h"
+#include "homos.h"
+#include "planar.h"
 
 #undef PACKAGE
 #undef PACKAGE_BUGREPORT
diff --git a/src/digraphs.h b/src/digraphs.h
index fad324269..b28c1c4be 100644
--- a/src/digraphs.h
+++ b/src/digraphs.h
@@ -15,14 +15,8 @@
 #ifndef DIGRAPHS_SRC_DIGRAPHS_H_
 #define DIGRAPHS_SRC_DIGRAPHS_H_
 
-#include <digraphs-debug.h>
-#include <stdbool.h>
-#include <stdlib.h>
-
-#include "bliss-0.73/bliss_C.h"
-
-#include "src/compiled.h" /* GAP headers                */
-#include "src/homos.h"
+// GAP headers
+#include "src/compiled.h"  // for Obj, Int
 
 Int DigraphNrVertices(Obj digraph);
 Int DigraphNrEdges(Obj digraph);
diff --git a/src/homos.c b/src/homos.c
index ebcc13fcf..96b0ff83e 100644
--- a/src/homos.c
+++ b/src/homos.c
@@ -9,8 +9,17 @@
 **
 ********************************************************************************/
 
-#include "src/homos.h"
-#include "src/digraphs-config.h"
+#include "homos.h"
+
+// C headers
+#include <setjmp.h>  // for setjmp
+
+// Bliss headers
+#include "bliss-0.73/bliss_C.h"  // for BlissGraph, . . .
+
+// Digraphs package headers
+#include "digraphs-debug.h"  // for DIGRAPHS_ASSERT
+#include "schreier-sims.h"   // for PermColl, . . .
 
 ////////////////////////////////////////////////////////////////////////////////
 ////////////////////////////////////////////////////////////////////////////////
@@ -109,33 +118,6 @@ static void init_mask(void) {
 ////////////////////////////////////////////////////////////////////////////////
 ////////////////////////////////////////////////////////////////////////////////
 
-////////////////////////////////////////////////////////////////////////////////
-// COUNT_TRUES_BLOCK: this is from gap/src/blister.h, it counts the number of
-// bits in a block which are set to 1.
-////////////////////////////////////////////////////////////////////////////////
-
-static inline UInt COUNT_TRUES_BLOCK(UInt block) {
-#if USE_POPCNT && defined(HAVE___BUILTIN_POPCOUNTL)
-  return __builtin_popcountl(block);
-#else
-#if SIZEOF_VOID_P == 8
-  block = (block & 0x5555555555555555L) + ((block >> 1) & 0x5555555555555555L);
-  block = (block & 0x3333333333333333L) + ((block >> 2) & 0x3333333333333333L);
-  block = (block + (block >> 4)) & 0x0f0f0f0f0f0f0f0fL;
-  block = (block + (block >> 8));
-  block = (block + (block >> 16));
-  block = (block + (block >> 32)) & 0x00000000000000ffL;
-#else
-  block = (block & 0x55555555) + ((block >> 1) & 0x55555555);
-  block = (block & 0x33333333) + ((block >> 2) & 0x33333333);
-  block = (block + (block >> 4)) & 0x0f0f0f0f;
-  block = (block + (block >> 8));
-  block = (block + (block >> 16)) & 0x000000ff;
-#endif
-  return block;
-#endif
-}
-
 ////////////////////////////////////////////////////////////////////////////////
 // new_bit_array: get a pointer to a new BitArray with space for <nr_bits>
 // bits, and with every bit set to false.
@@ -597,8 +579,7 @@ inline static bool is_adjacent_digraph(Digraph* digraph, Vertex i, Vertex j) {
 
 ////////////////////////////////////////////////////////////////////////////////
 // new_bliss_digraphs_graph: get a new bliss (undirected, vertex coloured)
-// digraph from
-// the Digraph pointed to by <digraph>.
+// digraph from the Digraph pointed to by <digraph>.
 ////////////////////////////////////////////////////////////////////////////////
 
 static BlissGraph* new_bliss_digraphs_graph_from_digraph(Digraph* digraph,
diff --git a/src/homos.h b/src/homos.h
index c8a59d76f..bc03dec55 100644
--- a/src/homos.h
+++ b/src/homos.h
@@ -12,11 +12,11 @@
 #ifndef DIGRAPHS_SRC_HOMOS_H_
 #define DIGRAPHS_SRC_HOMOS_H_
 
-#include <limits.h>
-#include <setjmp.h>
+// GAP headers
+#include "src/compiled.h"
 
-#include "bliss-0.73/bliss_C.h"
-#include "src/schreier-sims.h"
+// Digraphs package headers
+#include "perms.h"  // for UIntS
 
 void          homo_hook_print();
 typedef UIntS Vertex;
diff --git a/src/perms.c b/src/perms.c
index 686e23d5c..8dc831d21 100644
--- a/src/perms.c
+++ b/src/perms.c
@@ -11,7 +11,15 @@
 **  This file is free software, see the digraphs/LICENSE.
 **
 *******************************************************************************/
-#include "src/perms.h"
+
+#include "perms.h"
+
+// C headers
+#include <stdlib.h>  // for malloc, . . .
+#include <string.h>  // memcpy
+
+// Digraphs package headers
+#include "digraphs-debug.h"  // for DIGRAPHS_ASSERT
 
 // variables for debugging memory leaks
 UIntL nr_ss_allocs;
diff --git a/src/perms.h b/src/perms.h
index f30f31ab4..df33dd8bc 100644
--- a/src/perms.h
+++ b/src/perms.h
@@ -15,11 +15,7 @@
 #ifndef DIGRAPHS_SRC_PERMS_H_
 #define DIGRAPHS_SRC_PERMS_H_
 
-#include <digraphs-debug.h>
-#include <stdbool.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
+#include <stdbool.h>  // for bool
 
 #define MAXVERTS 512
 #define UNDEFINED MAXVERTS + 1
diff --git a/src/schreier-sims.c b/src/schreier-sims.c
index c68853122..9a9edafda 100644
--- a/src/schreier-sims.c
+++ b/src/schreier-sims.c
@@ -12,7 +12,14 @@
 **
 *******************************************************************************/
 
-#include "src/schreier-sims.h"
+#include "schreier-sims.h"
+
+// C headers
+#include "stdlib.h"  // for NULL
+#include "string.h"  // for memset
+
+// Digraphs package headers
+#include "digraphs-debug.h"  // for DIGRAPHS_ASSERT
 
 // Schreier-Sims set up
 
diff --git a/src/schreier-sims.h b/src/schreier-sims.h
index 777724c14..85fd7d88c 100644
--- a/src/schreier-sims.h
+++ b/src/schreier-sims.h
@@ -15,7 +15,7 @@
 #ifndef DIGRAPHS_SRC_SCHREIER_SIMS_H_
 #define DIGRAPHS_SRC_SCHREIER_SIMS_H_
 
-#include "src/perms.h"
+#include "perms.h"  // for PermColl and UIntS
 
 extern bool point_stabilizer(PermColl* gens, UIntS const pt, PermColl** out);