From ca2741f8d02d88288dfd1127c0faa9709ab83e6e Mon Sep 17 00:00:00 2001
From: Aleksander Sadikov <aleksander.sadikov@fri.uni-lj.si>
Date: Wed, 31 Aug 2016 03:08:54 +0200
Subject: English translation for permute/2 added.

---
 prolog/problems/lists/permute_2/en.py | 78 ++++++++++++++++++++++++++++++++++-
 1 file changed, 77 insertions(+), 1 deletion(-)

(limited to 'prolog/problems/lists/permute_2/en.py')

diff --git a/prolog/problems/lists/permute_2/en.py b/prolog/problems/lists/permute_2/en.py
index 201d8d6..64203f4 100644
--- a/prolog/problems/lists/permute_2/en.py
+++ b/prolog/problems/lists/permute_2/en.py
@@ -13,4 +13,80 @@ description = '''\
   L = [3,2,1].
 </pre>'''
 
-hint = {}
+plan = ['''\
+<p><img src="[%@resource plan.svg%]" /></p>
+<p>Try recursively constructing one possibility, but leave prolog its freedom. It will then return
+all the possibilities by itself.</p>
+''', '''\
+<p>There are several options, one goes like this: if I take away the head, this reduces the problem and I can
+leave it to recursion, and at the end I put the head back. Where do I put the head? I insert it in
+<em>all</em> possible places, one solution at a time.</p>
+''', '''\
+<p>If I assume the recursion permutes the tail <code>T</code> into the "permutated tail"
+<code>PT</code> and then I insert head <code>H</code> somewhere into <code>PT</code>, then I will get
+some permutation of the initial list <code>[H|T]</code>.</p>
+''']
+
+hint = {
+    'eq_instead_of_equ': '''\
+<p>The operator <code>==</code> is "stricter" than operator <code>=</code> in the sense that
+for the latter it is enough to be able to make the two operands equal (unification).</p>
+<p>Of course, you can also solve the exercise without explicit use of either of these two operators, just
+remember that unification is implicitly performed with the predicate's arguments (head of clause).</p>
+''',
+
+    'eq_instead_of_equ_markup': '''\
+<p>Perhaps the operator for unification (=) would be better?</p>
+''',
+
+    'base_case': '''\
+<p>Did you think of a base case? What's the most elementary case? What if the list is empty?</p>
+''',
+
+    'base_case_arbitrary': '''\
+<p>How can the result of permutating a list be an arbitrary list or an unassigned variable?</p>
+<p>If your base case is reminiscent of <code>permute([], _)</code>, rethink it!
+What should be the result of permutating a list?
+The base case <em>always</em> fully specifies the result, usually there are no unknowns (<code>_</code>
+or variables without assigned values) in what is being returned as the result.</p>
+''',
+
+    'unsuccessful_conc_use': '''\
+<p>Are you using <code>conc/3</code>? It's probably not so useful to solve this exercise,
+but the solution to some other previous exercise might be what is required.</p>
+''',
+
+    'forcing_result_onto_recursion': '''
+<p>Don't force the result onto recursion, don't tell it what it should return. Just let it be and
+assume it will do its job. If this assumption is correct, then the rule will work for a larger case.</p>
+<p>Is your recursive call of the form <code>permute(T, [H|...])</code>? This forces the recursive call to
+also <em>return</em> the head <code>H</code> which it <em>doesn't know of</em> since you previously took it away.
+Inserting the head into the result, returned by the recursive call, is your job. To put it shortly,
+insert <code>H</code> outside of the recursive call.</p>
+''',
+
+    'recursive_case': '''\
+<p>The base case is ok. However, what about the general recursive case?</p>
+''',
+
+    'no_insert_or_delete': '''\
+<p>The base case is ok. However, what about the general recursive case? Perhaps it's a good idea
+to reuse some previous exercise?</p>
+''',
+
+    'predicate_always_false': '''\
+<p>It seems your predicate is <emph>always</emph> "false". Did you give it the correct name,
+or is it perhaps misspelled?</p>
+<p>If the name is correct, check whether something else is misspelled, perhaps there is a full stop instead of
+a comma or vice versa, or maybe you typed a variable name in lowercase?</p>
+<p>It is, of course, also possible that your conditions are too restrictive, or even impossible to satisfy
+(as would be, for example, the condition that an empty list <code>[]</code> is equal to a list with
+exactly three elements <code>[A,B,C]</code>,
+or something similarly impossible).</p>
+''',
+
+    'timeout': '''\
+<p>Is there an infinite recursion at work here? How will it ever stop?</p>
+<p>Or perhaps is there a missing, faulty, or simply incompatible (with the general recursive case) base case?</p>
+''',
+}
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