diff --git a/filter/decl.m4 b/filter/decl.m4
index cc069485..20119c1d 100644
--- a/filter/decl.m4
+++ b/filter/decl.m4
@@ -6,149 +6,70 @@ m4_divert(-1)m4_dnl
 #
 #	Can be freely distributed and used under the terms of the GNU GPL.
 #
+#	THIS IS A M4 MACRO FILE GENERATING 3 FILES ALTOGETHER.
+#	KEEP YOUR HANDS OFF UNLESS YOU KNOW WHAT YOU'RE DOING.
+#	EDITING AND DEBUGGING THIS FILE MAY DAMAGE YOUR BRAIN SERIOUSLY.
 #
-#	Global Diversions:
-#	4	enum fi_code
-#	5	enum fi_code to string
-#	6	dump line item
-#	7	dump line item callers
-#	8	linearize
-#	9	same (filter comparator)
-#	1	union in struct f_inst
-#	3	constructors + interpreter
+#	But you're welcome to read and edit and debug if you aren't scared.
+#
+#	Uncomment the following line to get exhaustive debug output.
+#	m4_debugmode(aceflqtx)
+#
+#	How it works:
+#	1) Instruction to code conversion (uses diversions 100..199)
+#	2) Code wrapping (uses diversions 1..99)
+#	3) Final preparation (uses diversions 200..299)
+#	4) Shipout
+#
+#	See below for detailed description.
+#
+#
+#	1) Instruction to code conversion
+#	The code provided in f-inst.c between consecutive INST() calls
+#	is interleaved for many different places. It is here processed
+#	and split into separate instances where split-by-instruction
+#	happens. These parts are stored in temporary diversions listed:
 #
-#	Per-inst Diversions:
 #	101	content of per-inst struct
 #	102	constructor arguments
 #	103	constructor body
 #	104	dump line item content
+#		(there may be nothing in dump-line content and
+#		 it must be handled specially in phase 2)
 #	105	linearize body
 #	106	comparator body
 #	107	struct f_line_item content
 #	108	interpreter body
 #
-#	Final diversions
-#	200+	completed text before it is flushed to output
-
-m4_dnl m4_debugmode(aceflqtx)
-
-m4_define(FID_ZONE, `m4_divert($1) /* $2 for INST_NAME() */')
-m4_define(FID_INST, `FID_ZONE(1, Instruction structure for config)')
-m4_define(FID_LINE, `FID_ZONE(2, Instruction structure for interpreter)')
-m4_define(FID_NEW, `FID_ZONE(3, Constructor)')
-m4_define(FID_ENUM, `FID_ZONE(4, Code enum)')
-m4_define(FID_ENUM_STR, `FID_ZONE(5, Code enum to string)')
-m4_define(FID_DUMP, `FID_ZONE(6, Dump line)')
-m4_define(FID_DUMP_CALLER, `FID_ZONE(7, Dump line caller)')
-m4_define(FID_LINEARIZE, `FID_ZONE(8, Linearize)')
-m4_define(FID_SAME, `FID_ZONE(9, Comparison)')
-
+#	Here are macros to allow you to _divert to the right directions.
 m4_define(FID_STRUCT_IN, `m4_divert(101)')
 m4_define(FID_NEW_ARGS, `m4_divert(102)')
 m4_define(FID_NEW_BODY, `m4_divert(103)')
 m4_define(FID_DUMP_BODY, `m4_divert(104)m4_define([[FID_DUMP_BODY_EXISTS]])')
-m4_define(FID_LINEARIZE_BODY, `m4_divert(105)m4_define([[FID_LINEARIZE_BODY_EXISTS]])')
+m4_define(FID_LINEARIZE_BODY, `m4_divert(105)')
 m4_define(FID_SAME_BODY, `m4_divert(106)')
 m4_define(FID_LINE_IN, `m4_divert(107)')
 m4_define(FID_INTERPRET_BODY, `m4_divert(108)')
 
-m4_define(FID_ALL, `FID_INTERPRET_BODY');
+#	Sometimes you want slightly different code versions in different
+#	outputs.
+#	Use FID_HIC(code for inst-gen.h, code for inst-gen.c, code for inst-interpret.c)
+#	and put it into [[ ]] quotes if it shall contain commas.
 m4_define(FID_HIC, `m4_ifelse(TARGET, [[H]], [[$1]], TARGET, [[I]], [[$2]], TARGET, [[C]], [[$3]])')
 
+#	In interpreter code, this is quite common.
 m4_define(FID_INTERPRET_EXEC, `FID_HIC(,[[FID_INTERPRET_BODY()]],[[m4_divert(-1)]])')
 m4_define(FID_INTERPRET_NEW,  `FID_HIC(,[[m4_divert(-1)]],[[FID_INTERPRET_BODY()]])')
+
+#	If the instruction is never converted to constant, the interpret
+#	code is not produced at all for constructor
 m4_define(NEVER_CONSTANT, `m4_define([[INST_NEVER_CONSTANT]])')
 m4_define(FID_IFCONST, `m4_ifdef([[INST_NEVER_CONSTANT]],[[$2]],[[$1]])')
 
-m4_define(INST_FLUSH, `m4_ifdef([[INST_NAME]], [[
-FID_ENUM
-INST_NAME(),
-FID_ENUM_STR
-[INST_NAME()] = "INST_NAME()",
-FID_INST
-struct {
-m4_undivert(101)
-} i_[[]]INST_NAME();
-FID_LINE
-struct {
-m4_undivert(107)
-} i_[[]]INST_NAME();
-FID_NEW
-FID_HIC(
-[[
-struct f_inst *f_new_inst_]]INST_NAME()[[(enum f_instruction_code fi_code
-m4_undivert(102)
-);]],
-[[
-  case INST_NAME():
-  #define whati (&(what->i_]]INST_NAME()[[))
-  m4_ifelse(m4_eval(INST_INVAL() > 0), 1, [[if (fstk->vcnt < INST_INVAL()) runtime("Stack underflow"); fstk->vcnt -= INST_INVAL(); ]])
-  m4_undivert(108)
-  #undef whati
-  break;
-]],
-[[
-struct f_inst *f_new_inst_]]INST_NAME()[[(enum f_instruction_code fi_code
-m4_undivert(102)
-)
-  {
-    struct f_inst *what = fi_new(fi_code);
-    FID_IFCONST([[uint constargs = 1;]])
-  #define whati (&(what->i_]]INST_NAME()[[))
-  m4_undivert(103)
-    FID_IFCONST([[if (!constargs)]])
-      return what;
-    FID_IFCONST([[m4_undivert(108)]])
-  #undef whati
-  }
-]])
-
-FID_DUMP_CALLER
-case INST_NAME(): f_dump_line_item_]]INST_NAME()[[(item, indent + 1); break;
-
-FID_DUMP
-m4_ifdef([[FID_DUMP_BODY_EXISTS]],
-[[static inline void f_dump_line_item_]]INST_NAME()[[(const struct f_line_item *item_, const int indent)]],
-[[static inline void f_dump_line_item_]]INST_NAME()[[(const struct f_line_item *item UNUSED, const int indent UNUSED)]])
-m4_undefine([[FID_DUMP_BODY_EXISTS]])
-{
-#define item (&(item_->i_]]INST_NAME()[[))
-m4_undivert(104)
-#undef item
-}
-
-FID_LINEARIZE
-case INST_NAME(): {
-#define whati (&(what->i_]]INST_NAME()[[))
-#define item (&(dest->items[pos].i_]]INST_NAME()[[))
-  m4_undivert(105)
-#undef whati
-#undef item
-  dest->items[pos].fi_code = what->fi_code;
-  dest->items[pos].lineno = what->lineno;
-  break;
-}
-m4_undefine([[FID_LINEARIZE_BODY_EXISTS]])
-
-FID_SAME
-case INST_NAME():
-#define f1 (&(f1_->i_]]INST_NAME()[[))
-#define f2 (&(f2_->i_]]INST_NAME()[[))
-m4_undivert(106)
-#undef f1
-#undef f2
-break;
-m4_divert(-1)FID_FLUSH(101,200)
-]])')
-
-m4_define(INST, `m4_dnl
-INST_FLUSH()m4_dnl
-m4_define([[INST_NAME]], [[$1]])m4_dnl
-m4_define([[INST_INVAL]], [[$2]])m4_dnl
-m4_undefine([[INST_NEVER_CONSTANT]])m4_dnl
-FID_ALL() m4_dnl
-')
-
+#	If the instruction has some attributes (here called members),
+#	these are typically carried with the instruction from constructor
+#	to interpreter. This yields a line of code everywhere on the path.
+#	FID_MEMBER is a macro to help with this task.
 m4_define(FID_MEMBER, `m4_dnl
 FID_LINE_IN
 $1 $2;
@@ -170,8 +91,14 @@ debug("%s$4\n", INDENT, $5);
 ]])
 FID_INTERPRET_EXEC
 const $1 $2 = whati->$2
-FID_ALL')
+FID_INTERPRET_BODY')
 
+#	Instruction arguments are needed only until linearization is done.
+#	This puts the arguments into the filter line to be executed before
+#	the instruction itself.
+#
+#	To achieve this, ARG_ANY must be called before anything writes into
+#	the instruction line as it moves the instruction pointer forward.
 m4_define(ARG_ANY, `
 FID_STRUCT_IN
 struct f_inst * f$1;
@@ -188,14 +115,17 @@ FID_IFCONST([[
 }
 FID_LINEARIZE_BODY
 pos = linearize(dest, whati->f$1, pos);
-FID_ALL()')
+FID_INTERPRET_BODY()')
 
+#	Some arguments need to check their type. After that, ARG_ANY is called.
 m4_define(ARG, `ARG_ANY($1)
 FID_INTERPRET_EXEC()
 if (v$1.type != $2) runtime("Argument $1 of instruction %s must be of type $2, got 0x%02x", f_instruction_name(what->fi_code), v$1.type)m4_dnl
-FID_ALL()')
+FID_INTERPRET_BODY()')
 
-m4_define(LINEX, `FID_INTERPRET_EXEC()LINEX_($1)FID_INTERPRET_NEW()return $1 FID_ALL()')
+#	Executing another filter line. This replaces the recursion
+#	that was needed in the former implementation.
+m4_define(LINEX, `FID_INTERPRET_EXEC()LINEX_($1)FID_INTERPRET_NEW()return $1 FID_INTERPRET_BODY()')
 m4_define(LINEX_, `do {
   fstk->estk[fstk->ecnt].pos = 0;
   fstk->estk[fstk->ecnt].line = $1;
@@ -226,13 +156,16 @@ do { if (whati->fl$1) {
 } } while(0)
 FID_INTERPRET_NEW
 return whati->f$1
-FID_ALL()')
+FID_INTERPRET_BODY()')
 
+#	Some of the instructions have a result. These constructions
+#	state the result and put it to the right place.
 m4_define(RESULT, `RESULT_VAL([[ (struct f_val) { .type = $1, .val.$2 = $3 } ]])')
 m4_define(RESULT_VAL, `FID_HIC(, [[do { res = $1; fstk->vcnt++; } while (0)]],
 [[return fi_constant(what, $1)]])')
 m4_define(RESULT_VOID, `RESULT_VAL([[ (struct f_val) { .type = T_VOID } ]])')
 
+#	Some common filter instruction members
 m4_define(SYMBOL, `FID_MEMBER(struct symbol *, sym, 
 [[strcmp(f1->sym->name, f2->sym->name) || (f1->sym->class != f2->sym->class)]], symbol %s, item->sym->name)')
 m4_define(RTC, `FID_MEMBER(struct rtable_config *, rtc, [[strcmp(f1->rtc->name, f2->rtc->name)]], route table %s, item->rtc->name)')
@@ -240,13 +173,174 @@ m4_define(STATIC_ATTR, `FID_MEMBER(struct f_static_attr, sa, f1->sa.sa_code != f
 m4_define(DYNAMIC_ATTR, `FID_MEMBER(struct f_dynamic_attr, da, f1->da.ea_code != f2->da.ea_code,,)')
 m4_define(ACCESS_RTE, `NEVER_CONSTANT()')
 
+#	2) Code wrapping
+#	The code produced in 1xx temporary diversions is a raw code without
+#	any auxiliary commands and syntactical structures around. When the
+#	instruction is done, INST_FLUSH is called. More precisely, it is called
+#	at the beginning of INST() call and at the end of file.
+#
+#	INST_FLUSH picks all the temporary diversions, wraps their content
+#	into appropriate headers and structures and saves them into global
+#	diversions listed:
+#
+#	4	enum fi_code
+#	5	enum fi_code to string
+#	6	dump line item
+#	7	dump line item callers
+#	8	linearize
+#	9	same (filter comparator)
+#	1	union in struct f_inst
+#	3	constructors + interpreter
+#
+#	These global diversions contain blocks of code that can be directly
+#	put into the final file, yet it still can't be written out now as
+#	every instruction writes to all of these diversions.
+
+#	Code wrapping diversion names
+m4_define(FID_ZONE, `m4_divert($1) /* $2 for INST_NAME() */')
+m4_define(FID_INST, `FID_ZONE(1, Instruction structure for config)')
+m4_define(FID_LINE, `FID_ZONE(2, Instruction structure for interpreter)')
+m4_define(FID_NEW, `FID_ZONE(3, Constructor)')
+m4_define(FID_ENUM, `FID_ZONE(4, Code enum)')
+m4_define(FID_ENUM_STR, `FID_ZONE(5, Code enum to string)')
+m4_define(FID_DUMP, `FID_ZONE(6, Dump line)')
+m4_define(FID_DUMP_CALLER, `FID_ZONE(7, Dump line caller)')
+m4_define(FID_LINEARIZE, `FID_ZONE(8, Linearize)')
+m4_define(FID_SAME, `FID_ZONE(9, Comparison)')
+
+#	This macro does all the code wrapping. See inline comments.
+m4_define(INST_FLUSH, `m4_ifdef([[INST_NAME]], [[
+FID_ENUM			m4_dnl Contents of enum fi_code { ... }
+INST_NAME(),
+FID_ENUM_STR			m4_dnl Contents of const char * indexed by enum fi_code
+[INST_NAME()] = "INST_NAME()",
+FID_INST			m4_dnl Anonymous structure inside struct f_inst
+struct {
+m4_undivert(101)
+} i_[[]]INST_NAME();
+FID_LINE			m4_dnl Anonymous structure inside struct f_line_item
+struct {
+m4_undivert(107)
+} i_[[]]INST_NAME();
+FID_NEW				m4_dnl Constructor and interpreter code together
+FID_HIC(
+[[				m4_dnl Public declaration of constructor in H file
+struct f_inst *f_new_inst_]]INST_NAME()[[(enum f_instruction_code fi_code
+m4_undivert(102)
+);]],
+[[				m4_dnl The one case in The Big Switch inside interpreter
+  case INST_NAME():
+  #define whati (&(what->i_]]INST_NAME()[[))
+  m4_ifelse(m4_eval(INST_INVAL() > 0), 1, [[if (fstk->vcnt < INST_INVAL()) runtime("Stack underflow"); fstk->vcnt -= INST_INVAL(); ]])
+  m4_undivert(108)
+  #undef whati
+  break;
+]],
+[[				m4_dnl Constructor itself
+struct f_inst *f_new_inst_]]INST_NAME()[[(enum f_instruction_code fi_code
+m4_undivert(102)
+)
+  {
+    /* Allocate the structure */
+    struct f_inst *what = fi_new(fi_code);
+    FID_IFCONST([[uint constargs = 1;]])
+
+    /* Initialize all the members */
+  #define whati (&(what->i_]]INST_NAME()[[))
+  m4_undivert(103)
+
+    /* If not constant, return the instruction itself */
+    FID_IFCONST([[if (!constargs)]])
+      return what;
+
+    /* Try to pre-calculate the result */
+    FID_IFCONST([[m4_undivert(108)]])
+  #undef whati
+  }
+]])
+
+FID_DUMP_CALLER			m4_dnl Case in another big switch used in instruction dumping (debug)
+case INST_NAME(): f_dump_line_item_]]INST_NAME()[[(item, indent + 1); break;
+
+FID_DUMP			m4_dnl The dumper itself
+m4_ifdef([[FID_DUMP_BODY_EXISTS]],
+[[static inline void f_dump_line_item_]]INST_NAME()[[(const struct f_line_item *item_, const int indent)]],
+[[static inline void f_dump_line_item_]]INST_NAME()[[(const struct f_line_item *item UNUSED, const int indent UNUSED)]])
+m4_undefine([[FID_DUMP_BODY_EXISTS]])
+{
+#define item (&(item_->i_]]INST_NAME()[[))
+m4_undivert(104)
+#undef item
+}
+
+FID_LINEARIZE			m4_dnl The linearizer
+case INST_NAME(): {
+#define whati (&(what->i_]]INST_NAME()[[))
+#define item (&(dest->items[pos].i_]]INST_NAME()[[))
+  m4_undivert(105)
+#undef whati
+#undef item
+  dest->items[pos].fi_code = what->fi_code;
+  dest->items[pos].lineno = what->lineno;
+  break;
+}
+
+FID_SAME			m4_dnl This code compares two f_line"s while reconfiguring
+case INST_NAME():
+#define f1 (&(f1_->i_]]INST_NAME()[[))
+#define f2 (&(f2_->i_]]INST_NAME()[[))
+m4_undivert(106)
+#undef f1
+#undef f2
+break;
+
+m4_divert(-1)FID_FLUSH(101,200) m4_dnl And finally this flushes all the unused diversions
+]])')
+
+m4_define(INST, `m4_dnl				This macro is called on beginning of each instruction.
+INST_FLUSH()m4_dnl				First, old data is flushed
+m4_define([[INST_NAME]], [[$1]])m4_dnl		Then we store instruction name,
+m4_define([[INST_INVAL]], [[$2]])m4_dnl		instruction input value count
+m4_undefine([[INST_NEVER_CONSTANT]])m4_dnl	and reset NEVER_CONSTANT trigger.
+FID_INTERPRET_BODY() m4_dnl			By default, every code is interpreter code.
+')
+
+#	3) Final preparation
+#
+#	Now we prepare all the code around the global diversions.
+#	It must be here, not in m4wrap, as we want M4 to mark the code
+#	by #line directives correctly, not to claim that every single line
+#	is at the beginning of the m4wrap directive.
+#
+#	This part is split by the final file.
+#	H for inst-gen.h
+#	I for inst-interpret.c
+#	C for inst-gen.c
+#
+#	So we in cycle:
+#	  A. open a diversion
+#	  B. send there some code
+#	  C. close that diversion
+#	  D. flush a global diversion
+#	  E. open another diversion and goto B.
+#
+#	Final diversions
+#	200+	completed text before it is flushed to output
+
+#	This is a list of output diversions
 m4_define(FID_WR_PUT_LIST)
+
+#	This macro does the steps C to E, see before.
 m4_define(FID_WR_PUT_ALSO, `m4_define([[FID_WR_PUT_LIST]],FID_WR_PUT_LIST()[[FID_WR_DPUT(]]FID_WR_DIDX[[)FID_WR_DPUT(]]$1[[)]])m4_define([[FID_WR_DIDX]],m4_eval(FID_WR_DIDX+1))m4_divert(FID_WR_DIDX)')
 
+#	These macros do the splitting between H/I/C
 m4_define(FID_WR_DIRECT, `m4_ifelse(TARGET,[[$1]],[[FID_WR_INIT()]],[[FID_WR_STOP()]])')
 m4_define(FID_WR_INIT, `m4_define([[FID_WR_DIDX]],200)m4_define([[FID_WR_PUT]],[[FID_WR_PUT_ALSO($]][[@)]])m4_divert(200)')
 m4_define(FID_WR_STOP, `m4_define([[FID_WR_PUT]])m4_divert(-1)')
 
+#	Here is the direct code to be put into the output files
+#	together with the undiversions, being hidden under FID_WR_PUT()
+
 m4_changequote([[,]])
 FID_WR_DIRECT(I)
 FID_WR_PUT(3)
@@ -412,13 +506,24 @@ struct f_line_item {
 
 /* Instruction constructors */
 FID_WR_PUT(3)
-
 m4_divert(-1)
+
+#	4) Shipout
+#
+#	Everything is prepared in FID_WR_PUT_LIST now. Let's go!
+
 m4_changequote(`,')
 
+#	Flusher auxiliary macro
 m4_define(FID_FLUSH, `m4_ifelse($1,$2,,[[m4_undivert($1)FID_FLUSH(m4_eval($1+1),$2)]])')
+
+#	Defining the macro used in FID_WR_PUT_LIST
 m4_define(FID_WR_DPUT, `m4_undivert($1)')
 
+#	After the code is read and parsed, we:
 m4_m4wrap(`INST_FLUSH()m4_divert(0)FID_WR_PUT_LIST()m4_divert(-1)FID_FLUSH(1,200)')
 
 m4_changequote([[,]])
+#	And now M4 is going to parse f-inst.c, fill the diversions
+#	and after the file is done, the content of m4_m4wrap (see before)
+#	is executed.
diff --git a/filter/f-inst.c b/filter/f-inst.c
index edc97794..5f30ee38 100644
--- a/filter/f-inst.c
+++ b/filter/f-inst.c
@@ -167,7 +167,7 @@
 	}
 	whati->f1 = NULL;
       }
-    FID_ALL
+    FID_INTERPRET_BODY
 
     FID_INTERPRET_EXEC
     if (fstk->vcnt < whati->count) /* TODO: make this check systematic */
@@ -198,7 +198,7 @@
 
     FID_INTERPRET_EXEC
       fstk->vcnt -= whati->count;
-    FID_ALL
+    FID_INTERPRET_BODY
 
     pm->len = whati->count;
     RESULT(T_PATH_MASK, path_mask, pm);
@@ -337,7 +337,7 @@
     FID_LINEARIZE_BODY
     {
       uint opos = pos;
-      FID_ALL
+      FID_INTERPRET_BODY
 
     ARG_ANY(1);
 
@@ -345,7 +345,7 @@
       if (opos < pos)
 	dest->items[pos].flags |= FIF_PRINTED;
     }
-    FID_ALL
+    FID_INTERPRET_BODY
 
     FID_MEMBER(enum filter_return, fret, f1->fret != f2->fret, %s, filter_return_str(item->fret));
 
@@ -1045,7 +1045,8 @@
   INST(FI_ASSERT, 1, 0) {	/* Birdtest Assert */
     NEVER_CONSTANT;
     ARG(1, T_BOOL);
-    FID_MEMBER(char *, s, [[strcmp(f1->s, f2->s)]], string \"%s\", item->s);
+
+    FID_MEMBER(char *, s, [[strcmp(f1->s, f2->s)]], string %s, item->s);
 
     ASSERT(s);