7 files changed, 451 insertions, 20 deletions
diff --git a/prolog/problems/family_relations/aunt_2/en.py b/prolog/problems/family_relations/aunt_2/en.py
index 4314a7a..f8bf35b 100644
--- a/prolog/problems/family_relations/aunt_2/en.py
+++ b/prolog/problems/family_relations/aunt_2/en.py
@@ -7,6 +7,81 @@ description = '''\
 ?- aunt(sally, X).
   X = vanessa ;
   X = patricia.
-</pre>'''
+</pre>
+<p><a target="_blank" href="[%@resource famrel.svg%]">Family trees</a> are
+described with predicates <code>parent/2</code>, <code>male/1</code>, and
+<code>female/1</code>.</p>
+'''
+
+plan = ['''\
+<p>Perhaps you can make use of the solution to one of the previous exercises?</p>
+<p><img src="[%@resource Prolog_aunt_06.svg%]" alt="My sibling has a child..." /></p>
+''', '''\
+<p>If <code>X</code> is a sister of the parent of <code>Y</code>,
+then <code>X</code> is the aunt of <code>Y</code>.</p>
+''', '''\
+<p><img src="[%@resource Prolog_aunt_07.svg%]" alt="A sister of Y's parent is the aunt of Y." /></p>
+''']
+
+hint = {
+    'x_and_y_mixed_up': '''\
+<p><code>X</code> should be the aunt of <code>Y</code>, not the other way around.</p>
+''',
+
+    'precedence_fail': '''\
+<p>Did you mix up the precedence of operators <em>or</em> & <em>and</em>?</p>
+<p>Operator <em>and</em> has higher precedence than operator <em>or</em>.
+If you wish to change that you can use brackets (as usual in mathematics).</p>
+''',
+
+    'x_must_have_sibling': '''\
+<p>An aunt usually has a sibling...</p>
+<p><img src="[%@resource Prolog_aunt_02.svg%]" /></p>
+''',
+
+    'x_must_be_female': '''\
+<p>An aunt is usually female.</p>
+<p><img src="[%@resource Prolog_aunt_05.svg%]" /></p>
+''',  # TODO: Loni should mark female gender on some aunt hints!
+
+    'y_must_have_parent': '''\
+<p>It would probably make sense if the nephew has a parent, right?</p>
+<p><img src="[%@resource Prolog_aunt_01.svg%]" /></p>
+''',
+
+    'aunt_vs_mother': '''\
+<p>How can an aunt also be the mother of <code>Y</code>?
+Did you forget to specify that an aunt and the parent of <code>Y</code> cannot be the same person?</p>
+<p><img src="[%@resource Prolog_aunt_04.svg%]" /></p>
+''',
+
+    'x_need_not_be_parent': '''\
+<p><code>X</code> doesn't need to have children to be somebody's aunt...</p>
+''',
+
+    'y_need_not_be_parent': '''\
+<p>A nephew <code>Y</code> doesn't need to have children...</p>
+''',
+
+    'nephews_parent_can_be_male': '''\
+<p>A parent of nephew <code>Y</code> can also be male
+(which means that this parent cannot be a sister of <code>X</code>).</p>
+''',
+
+    'predicate_always_false': '''\
+<p>It seems your predicate is <em>always</em> "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 <code>X</code> is both a parent and a sister of <code>Y</code>, or
+something similarly impossible).</p>
+''',
+
+    'interesting_tidbit': '''\
+<p>Interesting tidbit. This exercise could also be solved using the solution to the <code>sister/2</code>
+relation. An aunt is a sister of some parent of <code>Y</code>.</p>
+<p><img src="[%@resource Prolog_aunt_07.svg%]" /></p>
+''',    # TODO: only trigger if sister is not part of correct solution
+}
 
-hint = {}
diff --git a/prolog/problems/family_relations/brother_2/en.py b/prolog/problems/family_relations/brother_2/en.py
index 1295450..e0db1f5 100644
--- a/prolog/problems/family_relations/brother_2/en.py
+++ b/prolog/problems/family_relations/brother_2/en.py
@@ -2,11 +2,76 @@ name = 'brother/2'
 slug = 'the brother relation'
 
 description = '''\
-<p><code>brother(X, Y)</code>: <code>X</code> is a brother of <code>Y</code>.</p>
+<p><code>brother(X, Y)</code>: <code>X</code> is a (half)brother of <code>Y</code>.</p>
 <pre>
 ?- brother(jeffrey, X).
   X = william ;
   X = sally.
-</pre>'''
+</pre>
+<p><a target="_blank" href="[%@resource famrel.svg%]">Family trees</a> are
+described with predicates <code>parent/2</code>, <code>male/1</code>, and
+<code>female/1</code>.</p>
+'''
 
-hint = {}
+plan = [
+    '''\
+<p><img src="[%@resource Prolog_brother_01.svg%]" alt="A brother is male and has a common parent with Y." /></p>''',
+    '''\
+<p>If <code>X</code> is male and if <code>X</code> and <code>Y</code> share a common parent
+then <code>X</code> is a brother of <code>Y</code>.</p>''',
+    '''\
+<p><img src="[%@resource Prolog_brother_02.svg%]" alt="Brother X is male and has
+at least one parent in common with Y." /></p>''',
+]
+
+hint = {
+    'x_y_must_be_different': [{'message': '''\
+<p><img src="[%@resource Prolog_brother_04.svg%]" /></p>
+<p>What if <code>X</code> and <code>Y</code> are the same person? Try the following query:</p>
+<pre>
+?- brother(william, Y).
+</pre>''', 'linkText': 'How can I check that <code>X</code> and <code>Y</code> are different?'},
+'''\
+<p>Using the operator <code>\==</code> I can check that <code>X</code> and <code>Y</code> are different.
+For example: <code>X \== Y</code></p>
+'''],
+
+    'x_must_be_male': '''\
+<p><img src="[%@resource Prolog_brother_03.svg%]" /></p>
+<p>A brother is usually male.</p>
+''',
+
+    'y_can_be_of_any_gender': '''\
+<p><code>Y</code> can actually be of <em>any</em> gender.</p>
+''',
+
+    'neq+_instead_of_neq': '''\
+<p>Use the operator \== instead of operators \= or =\=. It succeeds when the two operands are not the same,
+while \= succeeds if the operands <em>cannot be made</em> the same (cannot be unified), and =\= checks for
+arithmetic inequality and in this case both operands should be immediately arithmetically computable.</p>
+''',
+
+    'common_parent_needed': '''\
+<p><img src="[%@resource Prolog_brother_05.svg%]" /></p>
+<p>It would make sense if <code>X</code> and <code>Y</code> have at least one parent in common.</p>
+''',
+
+    'neq_used_too_early': '''\
+<p>Did you use the operator <code>\==</code> too early?</p>
+<p>Try moving it more towards the end of the rule. The problem is probably that the variables
+you are comparing don't yet have values assigned to them and are therefore different by default.
+Prolog doesn't check whether they become equal <em>later</em>, it only checks whether they're
+equal or not at the moment of comparison.</p>
+<p>Or maybe you're simply comparing the wrong two variables?</p>
+''',
+
+    'predicate_always_false': '''\
+<p>It seems your predicate is <em>always</em> "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 <code>X</code> is both a parent and a sister of <code>Y</code>, or
+something similarly impossible).</p>
+''',
+}
diff --git a/prolog/problems/family_relations/cousin_2/en.py b/prolog/problems/family_relations/cousin_2/en.py
index 4ca9c87..a5627ee 100644
--- a/prolog/problems/family_relations/cousin_2/en.py
+++ b/prolog/problems/family_relations/cousin_2/en.py
@@ -2,11 +2,78 @@ name = 'cousin/2'
 slug = 'the cousin relation'
 
 description = '''\
-<p><code>cousin(X, Y)</code>: <code>X</code> is a cousin (male or female) of <code>Y</code>.</p>
+<p><code>cousin(X, Y)</code>: <code>X</code> is a cousin of <code>Y</code>.</p>
 <pre>
 ?- cousin(andrew, X).
   X = vanessa ;
   X = patricia.
-</pre>'''
+</pre>
+<p><a target="_blank" href="[%@resource famrel.svg%]">Family trees</a> are
+described with predicates <code>parent/2</code>, <code>male/1</code>, and
+<code>female/1</code>.</p>
+'''
+
+plan = ['''\
+<p>Perhaps you can make use of the solution to one of the previous exercises?</p>
+<p><img src="[%@resource Prolog_cousin_01.svg%]" alt="I have a child, my sibling also has a child..." /></p>
+<p>But of course it's also possible to solve without any of the previous solutions...</p>
+''', '''\
+<p>If <code>PX</code> is a parent of <code>X</code> and
+<code>PY</code> is a parent of <code>Y</code> and if
+<code>PX</code> and <code>PY</code> are siblings,
+then <code>X</code> is a cousin to <code>Y</code>.</p>
+''', '''\
+<p><img src="[%@resource Prolog_cousin_03.svg%]" alt="I have a child, my sibling has a child, these two children
+are therefore..." /></p>
+''']
+
+hint = {
+    'gender_is_irrelevant': '''\
+<p>Is gender really important?</p>
+''',
+
+    'gender_is_irrelevant_markup': '''\
+<p>Is this necessary?</p>
+''',
+
+    'precedence_fail': '''\
+<p>Did you mix up the precedence of operators <em>or</em> & <em>and</em>?</p>
+<p>Operator <em>and</em> has higher precedence than operator <em>or</em>.
+If you wish to change that you can always use the brackets.</p>
+''',
+
+    'cousin_vs_sibling': '''\
+<p>How can a cousin <code>X</code> at the same time also be a sibling of <code>Y</code>?
+Did you perhaps forget that the parents of <code>X</code> and <code>Y</code> should not be the same?
+(Be careful: if both a mother and a father are listed in the database, then prolog could find them as two
+<em>different</em> parents, which is logically absolutely correct.)</p>
+<p><img src="[%@resource Prolog_cousin_04.svg%]" /></p>
+''',
+
+    'cousin_to_oneself': '''\
+<p>How can someone be a cousin to him- or herself?
+Perhaps <code>X</code> and <code>Y</code> have the same parent?</p>
+<p><img src="[%@resource Prolog_cousin_04.svg%]" /></p>
+<p>Try this query: <code>?- cousin(X, X).</code></p>
+''',
+
+    'cousin_need_not_be_parent': '''\
+<p>A cousin doesn't need to have children...</p>
+''',
+
+    'predicate_always_false': '''\
+<p>It seems your predicate is <em>always</em> "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 <code>X</code> is both a parent and a sister of <code>Y</code>, or
+something similarly impossible).</p>
+''',
+
+#     'interesting_tidbit': '''\
+# <p>Zanimivost: nalogo bi lahko rešil tudi z uporabo rešitve za relacijo "sister".
+# Teta je namreč sestra od starša od <code>Y</code>.</p>
+# ''',
+}
 
-hint = {}
diff --git a/prolog/problems/family_relations/father_2/en.py b/prolog/problems/family_relations/father_2/en.py
index b37c666..f1a9040 100644
--- a/prolog/problems/family_relations/father_2/en.py
+++ b/prolog/problems/family_relations/father_2/en.py
@@ -9,6 +9,57 @@ description = '''\
 ?- father(aleksander, X).
   X = luana ;
   X = daniela.
-</pre>'''
+</pre>
+<p><a target="_blank" href="[%@resource famrel.svg%]">Family trees</a> are
+described with predicates <code>parent/2</code>, <code>male/1</code>, and
+<code>female/1</code>.</p>
+'''
 
-hint = {}
+plan = [
+    '''\
+<p><img src="[%@resource Prolog_father_01.svg%]" alt="A father is male and has a child." /></p>''',
+    '''\
+<p><img src="[%@resource Prolog_father_02.svg%]" alt="Father X is a parent of Y and is male." /></p>''',
+    '''\
+<p>If <code>X</code> is male and <code>X</code> is parent to <code>Y</code>,
+then <code>X</code> is a father of <code>Y</code>.</p>'''
+]
+
+hint = {
+    'or_instead_of_and': '''\
+<p>Did you perhaps use a semicolon (that represents logical OR) instead of a comma (logical AND)?</p>
+''',
+
+    'or_instead_of_and_two_rules': '''\
+<p>Did you perhaps write two rules: one for gender and one for parenthood?
+Be careful: this is understood as one <em>or</em> the other, not as one <em>and</em> the other!</p>
+''',
+
+    'x_must_be_male': '''\
+<p>A father is usually male.</p>
+<p><img src="[%@resource Prolog_father_03.svg%]" /></p>
+''',
+
+    'x_must_be_parent': '''\
+<p>A father is supposed to have at least one child... so he's a parent of somebody.</p>
+<p><img src="[%@resource Prolog_father_04.svg%]" /></p>
+''',
+
+    'y_can_be_of_any_gender': '''\
+<p><code>Y</code> can be of any gender.</p>
+''',
+
+    'y_need_not_be_parent': '''\
+<p><code>Y</code> doesn't need to have children...</p>
+''',
+
+    'predicate_always_false': '''\
+<p>It seems your predicate is <em>always</em> "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 <code>X</code> is both a parent and a sister of <code>Y</code>, or
+something similarly impossible).</p>
+''',
+}
diff --git a/prolog/problems/family_relations/grandparent_2/en.py b/prolog/problems/family_relations/grandparent_2/en.py
index 3aa9bfe..b8fca7c 100644
--- a/prolog/problems/family_relations/grandparent_2/en.py
+++ b/prolog/problems/family_relations/grandparent_2/en.py
@@ -9,6 +9,64 @@ description = '''\
   X = patricia.
 ?- grandparent(tina, vanessa).
   true.
-</pre>'''
+</pre>
+<p><a target="_blank" href="[%@resource famrel.svg%]">Family trees</a> are
+described with predicates <code>parent/2</code>, <code>male/1</code>, and
+<code>female/1</code>.</p>
+'''
 
-hint = {}
+plan = ['''\
+<p><img src="[%@resource Prolog_grandparent_01.svg%]" alt="A grandparent has a child that has a child." /></p>
+''', '''\
+<p>If <code>X</code> is a parent of a parent of <code>Y</code>,
+then <code>X</code> is a grandparent of <code>Y</code>.</p>
+''', '''\
+<p><img src="[%@resource Prolog_grandparent_02.svg%]" alt="A grandparent has a child that has a child of
+his or her own." /></p>
+''']
+
+hint = {
+    'no_common_z': '''\
+<p>Did you connect <code>X</code> and <code>Y</code> through some common person?</p>
+''',
+
+    'gender_is_irrelevant': '''\
+<p>We're looking for a grandparent and a grandchild, the gender is not important.</p>
+''',
+
+    'gender_is_irrelevant_markup': '''\
+<p>Is this necessary?</p>
+''',
+
+    'or_instead_of_and': '''\
+<p>Did you perhaps use a semicolon (that represents logical OR) instead of a comma (logical AND)?</p>
+''',
+
+    'x_must_be_parent': '''\
+<p>A grandparent surely has a child, right? So he or she is a parent of someone...</p>
+<p><img src="[%@resource Prolog_grandparent_03.svg%]" </p>
+''',
+
+    'x_need_not_have_parent': '''\
+<p>A parent of <code>X</code> is not really important if we're looking for a grandchild...</p>
+''',
+
+    'y_must_have_parent': '''\
+<p>A grandchild <code>Y</code> must have a parent, right? And that parent is...</p>
+<p><img src="[%@resource Prolog_grandparent_04.svg%]" </p>
+''',
+
+    'y_need_not_be_parent': '''\
+<p>A grandchild <code>Y</code> doesn't really need to have children...</p>
+''',
+
+    'predicate_always_false': '''\
+<p>It seems your predicate is <em>always</em> "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 <code>X</code> is both a parent and a sister of <code>Y</code>, or
+something similarly impossible).</p>
+''',
+}
diff --git a/prolog/problems/family_relations/mother_2/en.py b/prolog/problems/family_relations/mother_2/en.py
index f8cda9a..7d9de60 100644
--- a/prolog/problems/family_relations/mother_2/en.py
+++ b/prolog/problems/family_relations/mother_2/en.py
@@ -9,6 +9,58 @@ description = '''\
 ?- mother(nevia, X).
   X = luana ;
   X = daniela.
-</pre>'''
+</pre>
+<p><a target="_blank" href="[%@resource famrel.svg%]">Family trees</a> are
+described with predicates <code>parent/2</code>, <code>male/1</code>, and
+<code>female/1</code>.</p>
+'''
 
-hint = {}
+plan = [
+    '''\
+<p><img src="[%@resource Prolog_mother_01.svg%]" alt="A mother is female and has a child." /></p>
+''',
+    '''\
+<p><img src="[%@resource Prolog_mother_02.svg%]" alt="Mother X is a parent of Y and is female." /></p>''',
+    '''\
+<p>If <code>X</code> is female and <code>X</code> is parent to <code>Y</code>,
+then <code>X</code> is a mother of <code>Y</code>.</p>'''
+]
+
+hint = {
+    'or_instead_of_and': '''\
+<p>Did you perhaps use a semicolon (that represents logical OR) instead of a comma (logical AND)?</p>
+''',
+
+    'or_instead_of_and_two_rules': '''\
+<p>Did you perhaps write two rules: one for gender and one for parenthood?
+Be careful: this is understood as one <em>or</em> the other, not as one <em>and</em> the other!</p>
+''',
+
+    'x_must_be_female': '''\
+<p>A mother is usually female.</p>
+<p><img src="[%@resource Prolog_mother_03.svg%]" /></p>
+''',
+
+    'x_must_be_parent': '''\
+<p>A mother is supposed to have at least one child... so she's a parent of somebody.</p>
+<p><img src="[%@resource Prolog_mother_04.svg%]" /></p>
+''',
+
+    'y_can_be_of_any_gender': '''\
+<p><code>Y</code> can be of any gender.</p>
+''',
+
+    'y_need_not_be_parent': '''\
+<p><code>Y</code> doesn't need to have children...</p>
+''',
+
+    'predicate_always_false': '''\
+<p>It seems your predicate is <em>always</em> "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 <code>X</code> is both a parent and a sister of <code>Y</code>, or
+something similarly impossible).</p>
+''',
+}
diff --git a/prolog/problems/family_relations/sister_2/en.py b/prolog/problems/family_relations/sister_2/en.py
index 9b78c95..cbd975b 100644
--- a/prolog/problems/family_relations/sister_2/en.py
+++ b/prolog/problems/family_relations/sister_2/en.py
@@ -2,18 +2,81 @@ name = 'sister/2'
 slug = 'the sister relation'
 
 description = '''\
-<p><code>sister(X, Y)</code>: <code>X</code> is a sister of <code>Y</code>.</p>
+<p><code>sister(X, Y)</code>: <code>X</code> is a (half)sister of <code>Y</code>.</p>
 <pre>
 ?- sister(vanessa, X).
   X = patricia.
-</pre>'''
+</pre>
+<p><a target="_blank" href="[%@resource famrel.svg%]">Family trees</a> are
+described with predicates <code>parent/2</code>, <code>male/1</code>, and
+<code>female/1</code>.</p>
+'''
+
+plan = [
+    '''\
+<p><img src="[%@resource Prolog_sister_01.svg%]" alt="A sister is female and has a common parent with Y." /></p>''',
+    '''\
+<p>If <code>X</code> is female and if <code>X</code> and <code>Y</code> share a common parent
+then <code>X</code> is a sister of <code>Y</code>.</p>''',
+    '''\
+<p><img src="[%@resource Prolog_sister_02.svg%]" alt="Sister X is female and has
+at least one parent in common with Y." /></p>''',
+]
 
 hint = {
-    'x_y_must_be_different': '''\
-<p>What if <code>X</code> and <code>Y</code> are the same person? Try the
-following query:</p>
+    'x_y_must_be_different': [{'message': '''\
+<p><img src="[%@resource Prolog_sister_04.svg%]" /></p>
+<p>What if <code>X</code> and <code>Y</code> are the same person? Try the following query:</p>
 <pre>
 ?- sister(sally, Y).
-</pre>
+</pre>''', 'linkText': 'How can I check that <code>X</code> and <code>Y</code> are different?'},
+'''\
+<p>Using the operator <code>\==</code> I can check that <code>X</code> and <code>Y</code> are different.
+For example: <code>X \== Y</code></p>
+'''],
+
+    'x_must_be_female': '''\
+<p><img src="[%@resource Prolog_sister_03.svg%]" /></p>
+<p>A sister is usually female.</p>
+''',
+
+    'y_can_be_of_any_gender': '''\
+<p><code>Y</code> can actually be of <em>any</em> gender.</p>
+''',
+
+    'neq+_instead_of_neq': '''\
+<p>Use the operator \== instead of operators \= or =\=. It succeeds when the two operands are not the same,
+while \= succeeds if the operands <em>cannot be made</em> the same (cannot be unified), and =\= checks for
+arithmetic inequality and in this case both operands should be immediately arithmetically computable.</p>
+''',
+
+    'common_parent_needed': '''\
+<p><img src="[%@resource Prolog_sister_05.svg%]" /></p>
+<p>It would make sense if <code>X</code> and <code>Y</code> have at least one parent in common.</p>
+''',
+
+    'neq_used_too_early': '''\
+<p>Did you use the operator <code>\==</code> too early?</p>
+<p>Try moving it more towards the end of the rule. The problem is probably that the variables
+you are comparing don't yet have values assigned to them and are therefore different by default.
+Prolog doesn't check whether they become equal <em>later</em>, it only checks whether they're
+equal or not at the moment of comparison.</p>
+<p>Or maybe you're simply comparing the wrong two variables?</p>
+''',
+
+    'predicate_always_false': '''\
+<p>It seems your predicate is <em>always</em> "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 <code>X</code> is both a parent and a sister of <code>Y</code>, or
+something similarly impossible).</p>
+''',
+
+    'final_hint': '''\
+<p>In this exercise you used the \== operator. It succeeds if its two operands are not the same
+<em>at the moment</em> of comparison. If you put this goal at the start of your rule, it will
+not work as expected. (Go ahead, try and see what happens!)</p>
 ''',
 }