diff --git a/C/heapgc.c b/C/heapgc.c
index 3696013ce..80673cbda 100644
--- a/C/heapgc.c
+++ b/C/heapgc.c
@@ -3492,10 +3492,6 @@ compaction_phase(tr_fr_ptr old_TR, CELL *current_env, yamop *curp, CELL *max)
       total_marked += total_oldies;
     }
   } else {
-    if (GcCalls==355) {
-      fprintf(stderr,"Start=%p,%pn %ld, %ld\n",H0,HGEN, total_oldies, total_marked);
-      
-    }
     if (HGEN != H0) {
       CurrentH0 = H0;
       H0 = HGEN;
diff --git a/changes-5.1.html b/changes-5.1.html
index 3099fbfc3..fec07757b 100644
--- a/changes-5.1.html
+++ b/changes-5.1.html
@@ -17,6 +17,7 @@
 
 <h2>Yap-5.1.3:</h2>
 <ul>
+<li> NEW: support conditional compilation using if/1,elif/1,else/0,endif/0 directives.</li>
 <li> FIXED: delay abort until garbage collection or stack shifting is over.</li>
 <li> FIXED: grow_trail assumed SREG points to stack top!</li>
 <li> FIXED: latest SWI chr.</li>
diff --git a/docs/yap.tex b/docs/yap.tex
index 6da53d26b..05a82e10b 100644
--- a/docs/yap.tex
+++ b/docs/yap.tex
@@ -160,6 +160,7 @@ Subnodes of Encoding
 Subnodes of Loading Programs
 * Compiling:: Program Loading and Updating
 * Setting the Compiler:: Changing the compiler's parameters
+* Conditional Compilation:: Compiling program fragments
 * Saving:: Saving and Restoring Programs
 
 Subnodes of Modules
@@ -1535,6 +1536,7 @@ writing, writing a BOM can be requested using the option
 Loading Programs
 * Compiling:: Program Loading and Updating
 * Setting the Compiler:: Changing the compiler's parameters
+* Conditional Compilation:: Compiling program fragments
 * Saving:: Saving and Restoring Programs
 
 @end menu
@@ -1616,7 +1618,7 @@ files specified by @var{F} into the file being currently consulted.
 
 @end table
 
-@node Setting the Compiler, Saving, Compiling, Loading Programs
+@node Setting the Compiler, Conditional Compilation, Compiling, Loading Programs
 @section Changing the Compiler's Behavior
 
 This section presents a set of built-ins predicates designed to set the 
@@ -1869,7 +1871,85 @@ Since YAP4.3.0 multifile procedures can be static or dynamic.
 
 @end table
 
-@node Saving, , Setting the Compiler, Loading Programs
+@node Conditional Compilation, Saving, Setting the Compiler, Loading Programs
+
+@section Conditional Compilation 
+
+@c \index{if, directive}%
+Conditional compilation builds on the same principle as
+@code{term_expansion/2}, @code{goal_expansion/2} and the expansion of
+grammar rules to compile sections of the source-code
+conditionally. One of the reasons for introducing conditional
+compilation is to simplify writing portable code.
+@c  See \secref{dialect}
+@c for more information. Here is a simple example:
+
+@c @table code
+@c :- if(\+source_exports(library(lists), suffix/2)).
+
+@c suffix(Suffix, List) :-
+@c 	append(_, Suffix, List).
+
+@c :- endif.
+@c \end{code}
+
+Note that these directives can only be appear as separate terms in the
+input.  Typical usage scenarios include:
+
+@itemize @bullet
+    @item Load different libraries on different dialects
+    @item Define a predicate if it is missing as a system predicate
+    @item Realise totally different implementations for a particular
+    part of the code due to different capabilities.
+    @item Realise different configuration options for your software.
+@end itemize
+
+
+@table @code
+@item if(+@var{Goal})
+@findex if/1 directive
+@snindex if/1
+@cnindex if/1
+Compile subsequent code only if @var{Goal} succeeds.  For enhanced
+portability, @var{Goal} is processed by @code{expand_goal/2} before execution.
+If an error occurs, the error is printed and processing proceeds as if
+@var{Goal} has failed.
+
+@item else
+@findex else/0 directive
+@snindex else/0
+@cnindex else/0
+Start `else' branch.
+
+@item endif
+@findex endif/0 directive
+@snindex endif/0
+@cnindex endif/0
+End of conditional compilation.
+
+@item elif(+@var{Goal})
+@findex elif/1 directive
+@snindex elif/1
+@cnindex elif/1
+Equivalent to @code{:- else. :-if(Goal) ... :- endif.}  In a sequence
+as below, the section below the first matching elif is processed, If
+no test succeeds the else branch is processed.
+
+@example
+:- if(test1).
+section_1.
+:- elif(test2).
+section_2.
+:- elif(test3).
+section_3.
+:- else.
+section_else.
+:- endif.
+@end example
+
+@end table
+
+@node Saving, , Conditional Compilation, Loading Programs
 @section Saving and Loading Prolog States
 
 @table @code
diff --git a/pl/boot.yap b/pl/boot.yap
index 481d611f9..fbd246a5c 100644
--- a/pl/boot.yap
+++ b/pl/boot.yap
@@ -56,6 +56,8 @@ true :- true.
 	('$exit_undefp' -> true ; true),
 	prompt('  ?- '),
 	nb_setval('$break',0),
+	nb_setval('$if_level',0),
+	nb_setval('$endif',off),
 	% '$set_read_error_handler'(error), let the user do that
 	nb_setval('$debug',off),
 	nb_setval('$trace',off),
@@ -80,121 +82,121 @@ true :- true.
 	'$startup_reconsult',
 	'$startup_goals'.
 
- % Start file for yap
+% Start file for yap
 
- /*		I/O predicates						*/
+/*		I/O predicates						*/
 
- /* meaning of flags for '$write' is
+/* meaning of flags for '$write' is
 	  1	quote illegal atoms
 	  2	ignore operator declarations
 	  4	output '$VAR'(N) terms as A, B, C, ...
 	  8	use portray(_)
- */
+*/
 
- /* main execution loop							*/
- '$read_vars'(Stream,T,Mod,Pos,V) :-
-	 '$read'(true,T,Mod,V,Pos,Err,Stream),
-	 (nonvar(Err) ->
-	     '$print_message'(error,Err), fail
-	     ;
-	     true
-	 ).
+/* main execution loop							*/
+'$read_vars'(Stream,T,Mod,Pos,V) :-
+	'$read'(true,T,Mod,V,Pos,Err,Stream),
+	(nonvar(Err) ->
+	 '$print_message'(error,Err), fail
+	;
+	 true
+	).
 
- % reset alarms when entering top-level.
- '$enter_top_level' :-
-	 '$alarm'(0, 0, _, _),
-	 fail.
- '$enter_top_level' :-
-	 '$clean_up_dead_clauses',
-	 fail.
- '$enter_top_level' :-
-	 recorded('$restore_goal',G,R),
-	 erase(R),
-	 prompt(_,'   | '),
-	 '$system_catch'('$do_yes_no'((G->true),user),user,Error,user:'$Error'(Error)),
-	 fail.
- '$enter_top_level' :-
-	 nb_getval('$break',BreakLevel),
-	 (
-	   nb_getval('$trace',on)
-	 ->
-	   TraceDebug = trace
-	 ;
-	   nb_getval('$debug', on)
-	 ->
-	   TraceDebug = debug
-	 ;
-	   true
-	 ),
-	 '$print_message'(informational,prompt(BreakLevel,TraceDebug)),
-	 fail.
- '$enter_top_level' :-
-	 get_value('$top_level_goal',GA), GA \= [], !,
-	 set_value('$top_level_goal',[]),
-	 '$run_atom_goal'(GA),
-	 set_value('$live','$false').
- '$enter_top_level' :-
-	 prompt(_,'   ?- '),
-	 prompt('   | '),
-	 '$run_toplevel_hooks',
-	 '$read_vars'(user_input,Command,_,_,Varnames),
-	 nb_setval('$spy_gn',1),
-	 % stop at spy-points if debugging is on.
-	 nb_setval('$debug_run',off),
-	 nb_setval('$debug_zip',off),
-	 prompt(_,'   |: '),
-	 '$command'((?-Command),Varnames,top),
-	 '$sync_mmapped_arrays',
-	 set_value('$live','$false').
+% reset alarms when entering top-level.
+'$enter_top_level' :-
+	'$alarm'(0, 0, _, _),
+	fail.
+'$enter_top_level' :-
+	'$clean_up_dead_clauses',
+	fail.
+'$enter_top_level' :-
+	recorded('$restore_goal',G,R),
+	erase(R),
+	prompt(_,'   | '),
+	'$system_catch'('$do_yes_no'((G->true),user),user,Error,user:'$Error'(Error)),
+	fail.
+'$enter_top_level' :-
+	nb_getval('$break',BreakLevel),
+	(
+	 nb_getval('$trace',on)
+	->
+	 TraceDebug = trace
+	;
+	 nb_getval('$debug', on)
+	->
+	 TraceDebug = debug
+	;
+	 true
+	),
+	'$print_message'(informational,prompt(BreakLevel,TraceDebug)),
+	fail.
+'$enter_top_level' :-
+	get_value('$top_level_goal',GA), GA \= [], !,
+	set_value('$top_level_goal',[]),
+	'$run_atom_goal'(GA),
+	set_value('$live','$false').
+'$enter_top_level' :-
+	prompt(_,'   ?- '),
+	prompt('   | '),
+	'$run_toplevel_hooks',
+	'$read_vars'(user_input,Command,_,_,Varnames),
+	nb_setval('$spy_gn',1),
+		% stop at spy-points if debugging is on.
+	nb_setval('$debug_run',off),
+	nb_setval('$debug_zip',off),
+	prompt(_,'   |: '),
+	'$command'((?-Command),Varnames,top),
+	'$sync_mmapped_arrays',
+	set_value('$live','$false').
 
- '$startup_goals' :-
-	 get_value('$extend_file_search_path',P), P \= [],
-	 set_value('$extend_file_search_path',[]),
-	 '$extend_file_search_path'(P),
-	 fail.
- '$startup_goals' :-
-	 recorded('$startup_goal',G,_),
-	 '$current_module'(Module),
-	 '$system_catch'('$query'(once(G), []),Module,Error,user:'$Error'(Error)),
-	 fail.
- '$startup_goals' :-
-	 get_value('$init_goal',GA),
-	 GA \= [],
- 	 set_value('$init_goal',[]),
-	 '$run_atom_goal'(GA),
-	 fail.
- '$startup_goals' :-
-	 get_value('$myddas_goal',GA), GA \= [],
-	 set_value('$myddas_goal',[]),
-	 get_value('$myddas_user',User), User \= [],
-	 set_value('$myddas_user',[]),
-	 get_value('$myddas_db',Db), Db \= [],
-	 set_value('$myddas_db',[]),
-	 get_value('$myddas_host',HostT),
-	 ( HostT \= [] ->
-	     Host = HostT,
-	     set_value('$myddas_host',[])
-	 ;
-	     Host = localhost
-	 ),
-	 get_value('$myddas_pass',PassT),
-	 ( PassT \= [] ->
-	     Pass = PassT,
-	     set_value('$myddas_pass',[])
-	 ;
-	     Pass = ''
-	 ),
-	 use_module(library(myddas)),
-	 call(db_open(mysql,myddas,Host/Db,User,Pass)),
-	 '$myddas_import_all',
-	 fail.
- '$startup_goals'.
+'$startup_goals' :-
+	get_value('$extend_file_search_path',P), P \= [],
+	set_value('$extend_file_search_path',[]),
+	'$extend_file_search_path'(P),
+	fail.
+'$startup_goals' :-
+	recorded('$startup_goal',G,_),
+	'$current_module'(Module),
+	'$system_catch'('$query'(once(G), []),Module,Error,user:'$Error'(Error)),
+	fail.
+'$startup_goals' :-
+	get_value('$init_goal',GA),
+	GA \= [],
+	set_value('$init_goal',[]),
+	'$run_atom_goal'(GA),
+	fail.
+'$startup_goals' :-
+	get_value('$myddas_goal',GA), GA \= [],
+	set_value('$myddas_goal',[]),
+	get_value('$myddas_user',User), User \= [],
+	set_value('$myddas_user',[]),
+	get_value('$myddas_db',Db), Db \= [],
+	set_value('$myddas_db',[]),
+	get_value('$myddas_host',HostT),
+	( HostT \= [] ->
+	  Host = HostT,
+	  set_value('$myddas_host',[])
+	;
+	  Host = localhost
+	),
+	get_value('$myddas_pass',PassT),
+	( PassT \= [] ->
+	  Pass = PassT,
+	  set_value('$myddas_pass',[])
+	;
+	  Pass = ''
+	),
+	use_module(library(myddas)),
+	call(db_open(mysql,myddas,Host/Db,User,Pass)),
+	'$myddas_import_all',
+	fail.
+'$startup_goals'.
 
- '$startup_reconsult' :-
-	 get_value('$consult_on_boot',X), X \= [], !,
-	 set_value('$consult_on_boot',[]),
-	 '$do_startup_reconsult'(X).
- '$startup_reconsult'.
+'$startup_reconsult' :-
+	get_value('$consult_on_boot',X), X \= [], !,
+	set_value('$consult_on_boot',[]),
+	'$do_startup_reconsult'(X).
+'$startup_reconsult'.
 
  %
  % MYDDAS: Import all the tables from one database
@@ -241,26 +243,29 @@ true :- true.
  '$repeat'.
  '$repeat' :- '$repeat'.
 
- '$start_corouts' :- recorded('$corout','$corout'(Name,_,_),R), Name \= main, finish_corout(R),
-		 fail.
- '$start_corouts' :- 
-		 eraseall('$corout'),
-		 eraseall('$result'),
-		 eraseall('$actual'),
-		 fail.
- '$start_corouts' :- recorda('$actual',main,_),
-	 recordz('$corout','$corout'(main,main,'$corout'([],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[])),_Ref),
-	 recorda('$result',going,_).
+'$start_corouts' :-
+	recorded('$corout','$corout'(Name,_,_),R),
+	Name \= main,
+	finish_corout(R),
+	fail.
+'$start_corouts' :- 
+	eraseall('$corout'),
+	eraseall('$result'),
+	eraseall('$actual'),
+	fail.
+'$start_corouts' :- recorda('$actual',main,_),
+	recordz('$corout','$corout'(main,main,'$corout'([],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[])),_Ref),
+	recorda('$result',going,_).
 
- '$command'(C,VL,Con) :-
-	 '$access_yap_flags'(9,1), !,
+'$command'(C,VL,Con) :-
+	'$access_yap_flags'(9,1), !,
 	 '$execute_command'(C,VL,Con,C).
- '$command'(C,VL,Con) :-
-	 ( (Con = top ; var(C) ; C = [_|_])  ->  
-	 '$execute_command'(C,VL,Con,C), ! ;
-	 expand_term(C, EC),
-	 '$execute_commands'(EC,VL,Con,C)
-	 ).
+'$command'(C,VL,Con) :-
+	( (Con = top ; var(C) ; C = [_|_])  ->  
+	  '$execute_command'(C,VL,Con,C), ! ;
+	  expand_term(C, EC),
+	  '$execute_commands'(EC,VL,Con,C)
+	).
 
  %
  % Hack in case expand_term has created a list of commands.
@@ -289,6 +294,11 @@ true :- true.
  '$execute_command'(R,_,top,Source) :- db_reference(R), !,
 	 '$do_error'(type_error(callable,R),meta_call(Source)).
  '$execute_command'(end_of_file,_,_,_) :- !.
+ '$execute_command'(Command,_,_,_) :-
+	 nb_getval('$if_skip_mode',skip),
+	 \+ '$if_directive'(Command),
+	 !,
+	 fail.
  '$execute_command'((:-G),_,Option,_) :- !,
 	 '$current_module'(M),
 	 % allow user expansion
diff --git a/pl/consult.yap b/pl/consult.yap
index 8e97110c5..312c1891b 100644
--- a/pl/consult.yap
+++ b/pl/consult.yap
@@ -200,6 +200,8 @@ use_module(M,F,Is) :-
 '$csult'([F|L], M) :- '$consult'(F, M), '$csult'(L, M).
 
 '$do_lf'(ContextModule, Stream, InfLevel, _, Imports, SkipUnixComments, Reconsult, UseModule) :-
+	nb_getval('$if_level',OldIncludeLevel),
+	nb_setval('$if_level',0),
 	nb_getval('$system_mode', OldMode),
         ( OldMode == off -> '$enter_system_mode' ; true ),
 	'$record_loaded'(Stream, ContextModule),
@@ -246,6 +248,11 @@ use_module(M,F,Is) :-
 	set_value('$consulting',Old),
 	set_value('$consulting_file',OldF),
 	cd(OldD),
+	nb_setval('$if_level',OldIncludeLevel),
+	% surely, we were in run mode or we would not have included the file!
+	nb_setval('$if_skip_mode',run),
+	% back to include mode!
+	nb_setval('$if_level',OldIncludeLevel),
 	'$current_module'(Mod,OldModule),
 	'$bind_module'(Mod, UseModule),
 	'$import_to_current_module'(File, ContextModule, Imports),
@@ -758,3 +765,98 @@ absolute_file_name(File,Opts,TrueFileName) :-
 	NewName =.. [N,NFile],
 	'$find_in_path'(NewName, Opts, OFile, Goal).
 
+%
+% This is complicated because of embedded ifs.
+%
+'$if'(_,top) :- !, fail.
+'$if'(Goal,_) :-
+	nb_getval('$if_level',Level0),
+	Level is Level0 + 1,
+	nb_setval('$if_level',Level),
+	nb_getval('$endif',OldEndif),
+	nb_getval('$if_skip_mode',Mode),
+	nb_setval('$endif',elif(Level,OldEndif,Mode)),
+	fail.
+% we are in skip mode, ignore....
+'$if'(Goal,_) :-
+	nb_getval('$endif',elif(Level, OldEndif, skip)), !,
+	nb_setval('$endif',endif(Level, OldEndif, skip)).	
+% we are in non skip mode, check....
+'$if'(Goal,_) :-
+	('$if_call'(Goal)
+	    ->
+	 % we will execute this branch, and later enter skip
+	 nb_getval('$endif',elif(Level,OldEndif,Mode)),
+	 nb_setval('$endif',endif(Level,OldEndif,Mode))
+	;
+	 % we are now in skip, but can start an elif.
+	 nb_setval('$if_skip_mode',skip)
+	).
+
+'$else'(top) :- !, fail.
+'$else'(_) :-
+	nb_getval('$if_level',0), !,
+	'$do_error'(context_error(no_if),(:- else)).
+% we have done an if, so just skip
+'$else'(_) :-
+	nb_getval('$endif',endif(_,_,_)), !,
+	nb_setval('$if_skip_mode',skip).
+% we can try the elif
+'$else'(_) :-
+	nb_getval('$if_level',Level),
+	nb_getval('$endif',elif(Level,OldEndif,Mode)),
+	nb_setval('$endif',endif(Level,OldEndif,Mode)),
+	nb_setval('$if_skip_mode',run).
+
+'$elif'(_,top) :- !, fail.
+'$elif'(Goal,_) :-
+	nb_getval('$if_level',0),
+	'$do_error'(context_error(no_if),(:- elif(Goal))).
+% we have done an if, so just skip
+'$elif'(_,_) :-
+	 nb_getval('$endif',endif(_,_,_)), !,
+	 nb_setval('$if_skip_mode',skip).
+% we can try the elif
+'$elif'(Goal,_) :-
+	nb_getval('$if_level',Level),
+	nb_getval('$endif',elif(Level,OldEndif,Mode)),
+	('$if_call'(Goal)
+	    ->
+% we will not skip, and we will not run any more branches.
+	 nb_setval('$endif',endif(Level,OldEndif,Mode)),
+	 nb_setval('$if_skip_mode',run)
+	;
+% we will (keep) on skipping
+	 nb_setval('$if_skip_mode',skip)
+	).
+'$elif'(_,_).
+
+'$endif'(top) :- !, fail.
+'$endif'(_) :-
+% unmmatched endif.
+	nb_getval('$if_level',0),
+	'$do_error'(context_error(no_if),(:- endif)).
+'$endif'(_) :-
+% back to where you belong.
+	nb_getval('$if_level',Level),
+	nb_getval('$endif',Endif),
+	Level0 is Level-1,
+	nb_setval('$if_level',Level0),
+	arg(2,Endif,OldEndif),
+	arg(3,Endif,OldMode),
+	nb_setval('$endif',OldEndif),
+	nb_setval('$if_skip_mode',OldMode).
+
+
+'$if_call'(G) :-
+	catch('$eval_if'(G), E, (print_message(error, E), fail)).
+
+'$eval_if'(Goal) :-
+	expand_term(Goal,TrueGoal),
+	once(TrueGoal).
+
+'$if_directive'((:- if(_))).
+'$if_directive'((:- else)).
+'$if_directive'((:- elif(_))).
+'$if_directive'((:- endif)).
+
diff --git a/pl/directives.yap b/pl/directives.yap
index 100e812e1..a8a6133bf 100644
--- a/pl/directives.yap
+++ b/pl/directives.yap
@@ -50,6 +50,10 @@
 '$directive'(thread_local(_)).
 '$directive'(uncutable(_)).
 '$directive'(encoding(_)).
+'$directive'(if(_)).
+'$directive'(else).
+'$directive'(elif(_)).
+'$directive'(endif).
 
 '$exec_directives'((G1,G2), Mode, M) :- !,
 	'$exec_directives'(G1, Mode, M),
@@ -117,6 +121,14 @@
 	'$table'(PredSpec, M).
 '$exec_directive'(uncutable(PredSpec), _, M) :-
 	'$uncutable'(PredSpec, M).
+'$exec_directive'(if(Goal), Context, M) :-
+	'$if'(M:Goal, Context).
+'$exec_directive'(else, Context, _) :-
+	'$else'(Context).
+'$exec_directive'(elif(Goal), Context, M) :-
+	'$elif'(M:Goal, Context).
+'$exec_directive'(endif, Context, _) :-
+	'$endif'(Context).
 
 
 yap_flag(V,Out) :-