Prolog: update denotational_semantics problems

16 files changed, 107 insertions, 348 deletions

diff --git a/prolog/problems/denotational_semantics/algol_3/common.py b/prolog/problems/denotational_semantics/algol_3/common.py
deleted file mode 100644
index 8138484..0000000
--- a/prolog/problems/denotational_semantics/algol_3/common.py
+++ /dev/null
@@ -1,78 +0,0 @@
-id = 176
-number = 83
-visible = False
-facts = None
-
-solution = '''\
-algol(fun(S0,S,apply176([printout=[]|S0],S,Minstructs))) -->
-  [begin], instructs176(Minstructs), [end].
-
-instructs176(Minstr) --> instr176(Minstr).
-instructs176(fun(S0,S,
-  (apply176(S0,S1,Minstr),
-   apply176(S1,S,Minstructs)))) 
-  --> 
-  instr176(Minstr), instructs176(Minstructs).
-
-instr176(Massign) --> assign176(Massign).
-instr176(fun(S0,[printout = L1|S1],
-  (memb(X = V,S0),
-   del(printout = L0,S0,S1),
-   conc(L0,[V],L1))))
-  -->
-  [print(X)].
-instr176(fun(S0,S,
-  loop176(S0,Mcond,Minstructs,S)))
-  -->
-  [while], cond176(Mcond), [do,begin], instructs176(Minstructs), [end].
-
-assign176(fun(S0,[X = Value|S1], 
-  (apply176(S0,Value,Mexpr),
-   del(X = _,S0,S1)))) 
-  -->
-  var176(X), [:=], expr176(Mexpr).
-
-cond176( fun( S, TruthVal,
-  (apply176(S,Val1,ME1),
-   apply176(S,Val2,ME2),
-   (Val1 < Val2,!,TruthVal = true ; TruthVal = false))))
-  -->
-  expr176(ME1), [<], expr176(ME2).
-
-var176(X) --> [X], {atom(X)}.
-
-expr176(fun(S,Value,eval(Expr,S,Value))) -->
-  [Expr].
-
-apply176(In, Out, fun(In, Out, Goals)) :-
-  call(Goals).
-
-loop176( State0, Mcond, _, State0) :-
-  apply176( State0, false, Mcond), !.
-loop176( S0, Mcond, MBody, S) :- 
-  copy_term( MBody, MBodyCopy),
-  apply176( S0, S1, MBody),
-  loop176( S1, Mcond, MBodyCopy, S).
-
-eval176( N, _, N) :-
-  number176( N), !.
-eval176( X, State, Val) :- % A program variable
-  atom( X), !, 
-  memb( X = Val, State). 
-eval176( E1 + E2, State, Val) :- !,
-  eval176( E1, State, V1),
-  eval176( E2, State, V2),
-  Val is V1 + V2.
-eval176( E1 - E2, State, Val) :- !,
-  eval176( E1, State, V1),
-  eval176( E2, State, V2),
-  Val is V1 - V2.
-eval176( E1 * E2, State, Val) :- !,
-  eval176( E1, State, V1),
-  eval176( E2, State, V2),
-  Val is V1 * V2.
-eval176( E1 / E2, State, Val) :- !,
-  eval176( E1, State, V1),
-  eval176( E2, State, V2),
-  Val is V1 /  V2.
-'''
diff --git a/prolog/problems/denotational_semantics/algol_3/en.py b/prolog/problems/denotational_semantics/algol_3/en.py
deleted file mode 100644
index 3b91a47..0000000
--- a/prolog/problems/denotational_semantics/algol_3/en.py
+++ /dev/null
@@ -1,32 +0,0 @@
-name = 'algol/3'
-slug = 'interpreter for mini-algol'
-
-description = '''\
-<p>A DCG for mini-algol.</p>
-<pre>
-% apply a function to a starting state
-?- apply([a=2], Out, fun(_In, Out, eval(a+3, _In, Out))).
-  Out = 5.
-
-% a := a+b
-% b := a-b
-% a := a-b
-?- _Program = [begin,a,:=,a+b,b,:=,a-b,a,:=,a-b,end],
-    algol(_F, _Program, []),
-    apply([a=3,b=5], Output, _F).
-  Output = [a=5,b=3,printout=[]].
-
-% a := 0
-% while a < 10 do
-% begin
-%   print(a)
-%   a := a+1
-% end
-?- _Program = [begin,a,:=,0,while,a,<,10,do,begin,print(a),a,:=,a+1,end,end],
-    algol(_F, _Program, []),
-    apply([a=3], Output, _F).
-  Output = [a=10,printout=[0,1,2,3,4,5,6,7,8,9]].
-</pre>
-'''
-
-hint = {}
diff --git a/prolog/problems/denotational_semantics/algol_for_3/common.py b/prolog/problems/denotational_semantics/algol_for_3/common.py
deleted file mode 100644
index 1107308..0000000
--- a/prolog/problems/denotational_semantics/algol_for_3/common.py
+++ /dev/null
@@ -1,92 +0,0 @@
-id = 178
-number = 85
-visible = False
-facts = None
-
-solution = '''\
-algol_for(fun(S0,S,apply178([printout=[]|S0],S,Minstructs))) -->
-  [begin], instructs178(Minstructs), [end].
-
-instructs178(Minstr) --> instr178(Minstr).
-instructs178(fun(S0,S,
-  (apply178(S0,S1,Minstr),
-   apply178(S1,S,Minstructs)))) 
-  --> 
-  instr178(Minstr), instructs178(Minstructs).
-
-instr178(Massign) --> assign178(Massign).
-instr178(fun(S0,[printout = L1|S1],
-  (memb(X = V,S0),
-   del(printout = L0,S0,S1),
-   conc(L0,[V],L1))))
-  -->
-  [print(X)].
-instr178(fun(S0,S,
-  loop178(S0,Mcond,Minstructs,S)))
-  -->
-  [while], cond178(Mcond), [do,begin], instructs178(Minstructs), [end].
-instr178(fun(S0, S,
-  (apply178(S0, S1, Minit),
-   for178(S1, Mcond, Mstep, Mbody, S))))
-  -->
-  [for], assign178(Minit), [&], cond178(Mcond), [&], assign178(Mstep), [do, begin], instructs178(Mbody), [end].
-
-assign178(fun(S0,[X = Value|S1], 
-  (apply178(S0,Value,Mexpr),
-   del(X = _,S0,S1)))) 
-  -->
-  var178(X), [:=], expr178(Mexpr).
-
-cond178( fun( S, TruthVal,
-  (apply178(S,Val1,ME1),
-   apply178(S,Val2,ME2),
-   (Val1 < Val2,!,TruthVal = true ; TruthVal = false))))
-  -->
-  expr178(ME1), [<], expr178(ME2).
-
-var178(X) --> [X], {atom(X)}.
-
-expr178(fun(S,Value,eval(Expr,S,Value))) -->
-  [Expr].
-
-apply178(In, Out, fun(In, Out, Goals)) :-
-  call(Goals).
-
-loop178( State0, Mcond, _, State0) :-
-  apply178( State0, false, Mcond), !.
-loop178( S0, Mcond, MBody, S) :- 
-  copy_term( MBody, MBodyCopy),
-  apply178( S0, S1, MBody),
-  loop178( S1, Mcond, MBodyCopy, S).
-
-for178(S0, Mcond, _, _, S0) :-
-  apply178(S0, false, Mcond), !.
-for178(S0, Mcond, Mstep, Mbody, S) :-
-  copy_term(Mbody, Mbody2),
-  copy_term(Mstep, Mstep2),
-  apply178(S0, S1, Mbody),
-  apply178(S1, S2, Mstep),
-  for178(S2, Mcond, Mstep2, Mbody2, S).
-
-eval178( N, _, N) :-
-  number178( N), !.
-eval178( X, State, Val) :- % A program variable
-  atom( X), !, 
-  memb( X = Val, State). 
-eval178( E1 + E2, State, Val) :- !,
-  eval178( E1, State, V1),
-  eval178( E2, State, V2),
-  Val is V1 + V2.
-eval178( E1 - E2, State, Val) :- !,
-  eval178( E1, State, V1),
-  eval178( E2, State, V2),
-  Val is V1 - V2.
-eval178( E1 * E2, State, Val) :- !,
-  eval178( E1, State, V1),
-  eval178( E2, State, V2),
-  Val is V1 * V2.
-eval178( E1 / E2, State, Val) :- !,
-  eval178( E1, State, V1),
-  eval178( E2, State, V2),
-  Val is V1 /  V2.
-'''
diff --git a/prolog/problems/denotational_semantics/algol_for_3/en.py b/prolog/problems/denotational_semantics/algol_for_3/en.py
deleted file mode 100644
index 2f46d36..0000000
--- a/prolog/problems/denotational_semantics/algol_for_3/en.py
+++ /dev/null
@@ -1,18 +0,0 @@
-name = 'algol_for/3'
-slug = 'interpreter for mini-algol with for-statement'
-
-description = '''\
-<p>Extend the given DCG for mini-algol to support the for-statement. Example:</p>
-<pre>
-% for a := 0 & a < 5 & a := a + 1 do
-% begin
-%   print(a)
-% end
-?- _Program = [begin,for,a,:=,0,&,a,<,5,&,a,:=,a+1,do,begin,print(a),end,end],
-    algol_for(_F, _Program, []),
-    apply([a=2], Output, _F).
-  Output = [a=5,printout=[0,1,2,3,4]].
-</pre>
-'''
-
-hint = {}
diff --git a/prolog/problems/denotational_semantics/algol_if_3/common.py b/prolog/problems/denotational_semantics/algol_if_3/common.py
deleted file mode 100644
index d98d2d2..0000000
--- a/prolog/problems/denotational_semantics/algol_if_3/common.py
+++ /dev/null
@@ -1,85 +0,0 @@
-id = 177
-number = 84
-visible = False
-facts = None
-
-solution = '''\
-algol_if(fun(S0,S,apply177([printout=[]|S0],S,Minstructs))) -->
-  [begin], instructs177(Minstructs), [end].
-
-instructs177(Minstr) --> instr177(Minstr).
-instructs177(fun(S0,S,
-  (apply177(S0,S1,Minstr),
-   apply177(S1,S,Minstructs)))) 
-  --> 
-  instr177(Minstr), instructs177(Minstructs).
-
-instr177(Massign) --> assign177(Massign).
-instr177(fun(S0,[printout = L1|S1],
-  (memb(X = V,S0),
-   del(printout = L0,S0,S1),
-   conc(L0,[V],L1))))
-  -->
-  [print(X)].
-instr177(fun(S0,S,
-  loop177(S0,Mcond,Minstructs,S)))
-  -->
-  [while], cond177(Mcond), [do,begin], instructs177(Minstructs), [end].
-instr177(fun(S0, S,
-  (apply177(S0, true, Mcond), !,
-   apply177(S0, S, MinstructsA)
-   ;
-   apply177(S0, S, MinstructsB))))
-  -->
-  [if], cond177(Mcond), [then], instructs177(MinstructsA), [else], instructs177(MinstructsB), [end].
-
-assign177(fun(S0,[X = Value|S1], 
-  (apply177(S0,Value,Mexpr),
-   del(X = _,S0,S1)))) 
-  -->
-  var177(X), [:=], expr177(Mexpr).
-
-cond177( fun( S, TruthVal,
-  (apply177(S,Val1,ME1),
-   apply177(S,Val2,ME2),
-   (Val1 < Val2,!,TruthVal = true ; TruthVal = false))))
-  -->
-  expr177(ME1), [<], expr177(ME2).
-
-var177(X) --> [X], {atom(X)}.
-
-expr177(fun(S,Value,eval(Expr,S,Value))) -->
-  [Expr].
-
-apply177(In, Out, fun(In, Out, Goals)) :-
-  call(Goals).
-
-loop177( State0, Mcond, _, State0) :-
-  apply177( State0, false, Mcond), !.
-loop177( S0, Mcond, MBody, S) :- 
-  copy_term( MBody, MBodyCopy),
-  apply177( S0, S1, MBody),
-  loop177( S1, Mcond, MBodyCopy, S).
-
-eval177( N, _, N) :-
-  number177( N), !.
-eval177( X, State, Val) :- % A program variable
-  atom( X), !, 
-  memb( X = Val, State). 
-eval177( E1 + E2, State, Val) :- !,
-  eval177( E1, State, V1),
-  eval177( E2, State, V2),
-  Val is V1 + V2.
-eval177( E1 - E2, State, Val) :- !,
-  eval177( E1, State, V1),
-  eval177( E2, State, V2),
-  Val is V1 - V2.
-eval177( E1 * E2, State, Val) :- !,
-  eval177( E1, State, V1),
-  eval177( E2, State, V2),
-  Val is V1 * V2.
-eval177( E1 / E2, State, Val) :- !,
-  eval177( E1, State, V1),
-  eval177( E2, State, V2),
-  Val is V1 /  V2.
-'''
diff --git a/prolog/problems/denotational_semantics/algol_if_3/en.py b/prolog/problems/denotational_semantics/algol_if_3/en.py
deleted file mode 100644
index bd5b732..0000000
--- a/prolog/problems/denotational_semantics/algol_if_3/en.py
+++ /dev/null
@@ -1,19 +0,0 @@
-name = 'algol_if/3'
-slug = 'interpreter for mini-algol with if-statement'
-
-description = '''\
-<p>Extend the given DCG for mini-algol to support the if-statement. You can assume that both branches are present in every if-statement. Example:</p>
-<pre>
-% if a < b then
-%   print(a)
-% else
-%   print(b)
-% end
-?- _Program = [begin,if,a,<,b,then,print(a),else,print(b),end,end],
-    algol_if(_F, _Program, []),
-    apply([a=3,b=5], Output, _F).
-  Output = [a=3,b=5,printout=[3]].
-</pre>
-'''
-
-hint = {}
diff --git a/prolog/problems/denotational_semantics/en.py b/prolog/problems/denotational_semantics/en.py
index 418b1e7..a245e15 100644
--- a/prolog/problems/denotational_semantics/en.py
+++ b/prolog/problems/denotational_semantics/en.py
@@ -1,2 +1,2 @@
 name = 'Denotational semantics'
-description = 'Adding meaning to grammars, writing a simple compiler for Algol-like programming language.'
+description = 'Adding meaning to grammars for list-processing languages.'
diff --git a/prolog/problems/denotational_semantics/prog_8puzzle_2/common.py b/prolog/problems/denotational_semantics/prog_8puzzle_2/common.py
index 71875d4..c5b69da 100644
--- a/prolog/problems/denotational_semantics/prog_8puzzle_2/common.py
+++ b/prolog/problems/denotational_semantics/prog_8puzzle_2/common.py
@@ -1,6 +1,6 @@
 id = 172
-number = 81
-visible = False
+number = 30
+visible = True
 facts = None
 
 solution = '''\
@@ -14,3 +14,15 @@ instr172 --> [right].
 instr172 --> [up].
 instr172 --> [down].
 '''
+
+initial = '''\
+prog_8puzzle --> [begin], instructs, [end].
+
+instructs --> instr.
+instructs --> instr, instructs.
+
+instr --> [left].
+instr --> [right].
+instr --> [up].
+instr --> [down].
+'''
diff --git a/prolog/problems/denotational_semantics/prog_8puzzle_2/en.py b/prolog/problems/denotational_semantics/prog_8puzzle_2/en.py
index 7969ba4..a91e22e 100644
--- a/prolog/problems/denotational_semantics/prog_8puzzle_2/en.py
+++ b/prolog/problems/denotational_semantics/prog_8puzzle_2/en.py
@@ -2,8 +2,17 @@ name = 'prog_8puzzle/2'
 slug = '8-puzzle-solving language'
 
 description = '''\
-<p>Write a DCG for solving 8-puzzles. The first symbol in every word is <code>[begin]</code>, followed by any sequence of "instruction" symbols from the set {<code>left</code>, <code>right</code>, <code>up</code>, <code>down</code>}, and finally <code>[end]</code>. The starting symbol should be named <code>prog_8puzzle</code>.</p>
+<p>
+Write a DCG for solving 8-puzzles. The first symbol in every word is
+<code>[begin]</code>, followed by any sequence of "instruction" symbols from
+the set {<code>left</code>, <code>right</code>, <code>up</code>,
+<code>down</code>}, and finally <code>[end]</code>. The starting symbol should
+be named <code>prog_8puzzle</code>.
+</p>
 
-<p>Example words: <code>[begin,left,down,right,end]</code>, <code>[begin,down,end]</code>.</p>'''
+<p>
+Example words: <code>[begin,left,down,right,end]</code>, <code>[begin,down,end]</code>.
+</p>
+'''
 
 hint = {}
diff --git a/prolog/problems/denotational_semantics/prog_8puzzle_3/common.py b/prolog/problems/denotational_semantics/prog_8puzzle_3/common.py
index 13debcf..0e0d048 100644
--- a/prolog/problems/denotational_semantics/prog_8puzzle_3/common.py
+++ b/prolog/problems/denotational_semantics/prog_8puzzle_3/common.py
@@ -1,6 +1,6 @@
 id = 173
-number = 82
-visible = False
+number = 40
+visible = True
 facts = 'denotational_semantics_aux__predicates'
 
 solution = '''\
diff --git a/prolog/problems/denotational_semantics/prog_8puzzle_3/en.py b/prolog/problems/denotational_semantics/prog_8puzzle_3/en.py
index 51aa81a..7a94c74 100644
--- a/prolog/problems/denotational_semantics/prog_8puzzle_3/en.py
+++ b/prolog/problems/denotational_semantics/prog_8puzzle_3/en.py
@@ -2,17 +2,38 @@ name = 'prog_8puzzle/3'
 slug = '8-puzzle-solving language with semantics'
 
 description = '''\
-<p>Write a DCG for solving 8-puzzles. The syntax for this language should be the same as in the previous exercise: the first symbol in every word is <code>[begin]</code>, followed by any sequence of "instruction" symbols from the set {<code>left</code>, <code>right</code>, <code>up</code>, <code>down</code>}, and finally <code>[end]</code>. The starting symbol should be named <code>prog_8puzzle</code>.</p>
+<p>
+Write a DCG for solving 8-puzzles. The syntax for this language should be the
+same as in the previous exercise: the first symbol in every word is
+<code>[begin]</code>, followed by any sequence of "instruction" symbols from
+the set {<code>left</code>, <code>right</code>, <code>up</code>,
+<code>down</code>}, and finally <code>[end]</code>. The starting symbol should
+be named <code>prog_8puzzle</code>.
+</p>
 
-<p>The meaning of a word (program) in this language has the form <code>In-->Out</code>, mapping from input to output states. Each state is a (permuted) list of numbers from 0 to 8, where 0 stands for the empty square and other numbers for the corresponding tiles. The first three numbers in the list correspond to the top row of the 8-puzzle, the next three numbers to the middle row, and the last three numbers to the last row. The meaning of instructions <code>left</code>, <code>right</code>, <code>up</code> and <code>down</code> is to move the blank tile in the given direction.</p>
+<p>
+The meaning of a word (program) in this language has the form
+<code>In-->Out</code>, mapping from input to output states. Each state is a
+(permuted) list of numbers from 0 to 8, where 0 stands for the empty square and
+other numbers for the corresponding tiles. The first three numbers in the list
+correspond to the top row of the 8-puzzle, the next three numbers to the middle
+row, and the last three numbers to the last row. The meaning of instructions
+<code>left</code>, <code>right</code>, <code>up</code> and <code>down</code> is
+to move the blank tile in the given direction.
+</p>
 
 <pre>
 ?- prog_8puzzle([0,1,2,3,4,5,6,7,8]-->Out, [begin,down,right,end], []).
   Out = [3,1,2,4,0,5,6,7,8].
 </pre>
 
-<p>Helper predicates (already defined):<br />
-&nbsp;&nbsp;<code>findblank(List,I)</code> returns the 1-based index <code>I</code> of the element 0 in <code>List</code><br />
-&nbsp;&nbsp;<code>swap(List,I,J,NewList)</code> creates <code>NewList</code> by swapping elements <code>I</code> and <code>J</code> in <code>List</code></p>'''
+<p>
+Helper predicates (already defined):
+</p>
+<ul>
+<li><code>findblank(List,I)</code> returns the 1-based index <code>I</code> of the element 0 in <code>List</code></li>
+<li><code>swap(List,I,J,NewList)</code> creates <code>NewList</code> by swapping elements <code>I</code> and <code>J</code> in <code>List</code></li>
+</ul>
+'''
 
 hint = {}
diff --git a/prolog/problems/denotational_semantics/prog_listswap_2/common.py b/prolog/problems/denotational_semantics/prog_listswap_2/common.py
index e08337d..b3da01f 100644
--- a/prolog/problems/denotational_semantics/prog_listswap_2/common.py
+++ b/prolog/problems/denotational_semantics/prog_listswap_2/common.py
@@ -1,10 +1,10 @@
 id = 175
-number = 79
-visible = False
+number = 10
+visible = True
 facts = None
 
 solution = '''\
-program --> [begin], instructs175, [end].
+prog_listswap --> [begin], instructs175, [end].
 
 instructs175 --> instr175.
 instructs175 --> instr175, instructs175.
@@ -13,3 +13,15 @@ instr175 --> [left].
 instr175 --> [right].
 instr175 --> [swap].
 '''
+
+# nothing to do in this exercise
+initial = '''\
+prog_listswap --> [begin], instructs, [end].
+
+instructs --> instr.
+instructs --> instr, instructs.
+
+instr --> [left].
+instr --> [right].
+instr --> [swap].
+'''
diff --git a/prolog/problems/denotational_semantics/prog_listswap_2/en.py b/prolog/problems/denotational_semantics/prog_listswap_2/en.py
index ad0e9b1..89907bd 100644
--- a/prolog/problems/denotational_semantics/prog_listswap_2/en.py
+++ b/prolog/problems/denotational_semantics/prog_listswap_2/en.py
@@ -2,9 +2,17 @@ name = 'prog_listswap/2'
 slug = 'list-manipulation language'
 
 description = '''\
-<p>Write a DCG for manipulating list elements. The first symbol in every word is <code>[begin]</code>, followed by any sequence of "instruction" symbols from the set {<code>left</code>, <code>right</code>, <code>swap</code>}, and finally <code>[end]</code>. The starting symbol should be named <code>prog_listswap</code>.</p>
+<p>
+Write a DCG for manipulating list elements. The first symbol in every word is
+<code>[begin]</code>, followed by any sequence of "instruction" symbols from
+the set {<code>left</code>, <code>right</code>, <code>swap</code>}, and finally
+<code>[end]</code>. The starting symbol should be named
+<code>prog_listswap</code>.
+</p>
 
-<p>Example words: <code>[begin,right,swap,end]</code>, <code>[begin,right,left,end]</code>.</p>
+<p>
+Example words: <code>[begin,right,swap,end]</code>, <code>[begin,right,left,end]</code>.
+</p>
 '''
 
 hint = {}
diff --git a/prolog/problems/denotational_semantics/prog_listswap_3/common.py b/prolog/problems/denotational_semantics/prog_listswap_3/common.py
index dfa7c86..3ad93e8 100644
--- a/prolog/problems/denotational_semantics/prog_listswap_3/common.py
+++ b/prolog/problems/denotational_semantics/prog_listswap_3/common.py
@@ -1,6 +1,6 @@
 id = 174
-number = 80
-visible = False
+number = 20
+visible = True
 facts = 'denotational_semantics_aux__predicates'
 
 solution = '''\
diff --git a/prolog/problems/denotational_semantics/prog_listswap_3/en.py b/prolog/problems/denotational_semantics/prog_listswap_3/en.py
index b8e9769..e275335 100644
--- a/prolog/problems/denotational_semantics/prog_listswap_3/en.py
+++ b/prolog/problems/denotational_semantics/prog_listswap_3/en.py
@@ -2,16 +2,37 @@ name = 'prog_listswap/3'
 slug = 'list-manipulation language with semantics'
 
 description = '''\
-<p>Write a DCG for manipulating list elements. The first symbol in every word is <code>[begin]</code>, followed by any sequence of "instruction" symbols from the set {<code>left</code>, <code>right</code>, <code>swap</code>}, and finally <code>[end]</code>. The starting symbol should be named <code>prog_listswap</code>.</p>
+<p>
+Write a DCG for manipulating list elements. The first symbol in every word is
+<code>[begin]</code>, followed by any sequence of "instruction" symbols from
+the set {<code>left</code>, <code>right</code>, <code>swap</code>}, and finally
+<code>[end]</code>. The starting symbol should be named
+<code>prog_listswap</code>.
+</p>
 
-<p>The meaning of a word (program) in this language has the form <code>In-->Out</code>, mapping from input to output lists. Besides the list contents, internal states also hold the current cursor position. The <code>left</code> and <code>right</code> instructions move the cursor one step in the given direction, while the <code>swap</code> instruction swaps the element under the cursor with its left neighbor (and fails if cursor is currently pointing to the first element of the list).</p>
+<p>
+The meaning of a word (program) in this language has the form
+<code>In-->Out</code>, mapping from input to output lists. Besides the list
+contents, internal states also hold the current cursor position. The
+<code>left</code> and <code>right</code> instructions move the cursor one step
+in the given direction, while the <code>swap</code> instruction swaps the
+element under the cursor with its left neighbor (and fails if cursor is
+currently pointing to the first element of the list).
+</p>
 
 <pre>
 ?- prog_listswap([1,2,3,4]-->Out, [begin,right,swap,end], []).
   Out = [2,1,3,4].
 </pre>
 
-<p>Helper predicate (already defined):<br />
-&nbsp;&nbsp;<code>swap(List,I,NewList)</code> creates <code>NewList</code> by swapping the <code>I</code>th element with its left neighbor in <code>List</code></p>'''
+<p>
+Helper predicates (already defined):
+</p>
+<ul>
+<li><code>swap(List,I,NewList)</code> creates <code>NewList</code> by
+swapping the <code>I</code>th element with its left neighbor in
+<code>List</code></li>
+</ul>
+'''
 
 hint = {}
diff --git a/prolog/problems/denotational_semantics/sl.py b/prolog/problems/denotational_semantics/sl.py
index d0e8f1e..6a7f823 100644
--- a/prolog/problems/denotational_semantics/sl.py
+++ b/prolog/problems/denotational_semantics/sl.py
@@ -1,2 +1,2 @@
 name = 'Denotacijske semantike'
-description = 'Dodajanje pomena gramatikam, pisanje preprostega prevajalnika za programski jezik algolskega tipa.'
+description = 'Dodajanje pomena gramatikam jezikov za delo s seznami.'