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+++ b/3-Prolog/day2-concat.pl	Wed Sep 27 20:48:28 2017 +0100
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+% Rewriting "append" from scratch as "concat"
+% Append/3 is true when List1+List2 = List3. This can be used to:
+%  * check whether two lists combine to a third (supply all args)
+%  * combine lists (supply first two and a variable for the third)
+%  * remove an item from a list (pass the first and third and the variable in 2)
+%  * computing splits of the list (pass two variables and a list - possibly the least useful, normally)
+%
+% The following is written without looking at the code, just the list of features.
+
+% Concat an empty list
+concatenate(List1, List2, List3) :- List2 = [], List3 = List1.
+% Concat one item from 1 on to 2
+concatenate(List1, List2, List3) :- List1 = [Head1], List3 = [Head1|List2].
+% Concat more items
+concatenate(List1, List2, List3) :- List1 = [Head1|Tail], concatenate(Tail, List2, TempTail), List3 = [Head1|TempTail].
+% See what we can generalise…
+% Erm, already seems rather general!
+%
+% Looking at the code, we can simplify by:
+%  1) making the first rule a fact (which I thought I might be able to do, but the wording implied I couldn't. And it worked so I left it)
+%  2) shifting the unifying from the right to the left
+%  3) taking advantage of "[Head|Tail]" matching an empty tail to combine the second two rules
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