Add GAP syntax highlight

Closes #2

episodes/01-command-line.md

@@ -47,13 +47,13 @@ including this one, must be finished with a semicolon! Practice entering
 may wish to enter the following command to display GAP prompts and user inputs
 in different colours:
 
-```source
+```gap
  ColorPrompt(true);
 ```
 
 The easiest way to start trying GAP out is as a calculator:
 
-```source
+```gap
 ( 1 + 2^32 ) / (1 - 2*3*107 );
 ```
 
@@ -64,7 +64,7 @@ The easiest way to start trying GAP out is as a calculator:
 If you want to record what you did in a GAP session, so you can look over it
 later, you can enable logging with the `LogTo` function, like this.
 
-```source
+```gap
 LogTo("gap-intro.log");
 ```
 
@@ -79,7 +79,7 @@ return to it in the future. A comment in GAP starts with the symbol `#` and
 continues to the end of the line. You can enter the following after the
 GAP prompt:
 
-```source
+```gap
 # GAP Software Carpentry Lesson
 ```
 
@@ -93,7 +93,7 @@ arrow keys to scroll the *command line history*. Repeat this until you see
 the formula again, then press Return (the location of the cursor in the command
 line does not matter):
 
-```source
+```gap
 ( 1 + 2^32 ) / (1 - 2*3*107 );
 ```
 
@@ -108,7 +108,7 @@ Ctrl-A and Ctrl-E to move the cursor to the beginning and end of the
 line, respectively). Now press the Return key (at any position of the
 cursor in the command line):
 
-```source
+```gap
 ( 1 + 2^64 ) / (1 - 2*3*107 );
 ```
 
@@ -124,7 +124,7 @@ the same string.
 If you want to store a value for later use, you can assign it to a name
 using `:=`
 
-```source
+```gap
 universe := 6*7;
 ```
 
@@ -140,15 +140,15 @@ universe := 6*7;
 
 - Finally, GAP uses `<>` to check if two things are not equal (rather than the `!=`
   you might have seen before).
-  
+
 
 ::::::::::::::::::::::::::::::::::::::::::::::::::
 
 Whitespace characters (i.e. Spaces, Tabs and Returns) are insignificant in GAP,
 except if they occur inside a string. For example, the previous input
 could be typed without spaces:
 
-```source
+```gap
 (1+2^64)/(1-2*3*107);
 ```
 
@@ -159,7 +159,7 @@ could be typed without spaces:
 Whitespace symbols are often used to format more complicated commands for
 better readability. For example, the following input which creates a 3×3 matrix:
 
-```source
+```gap
 m:=[[1,2,3],[4,5,6],[7,8,9]];
 ```
 
@@ -171,7 +171,7 @@ We can instead write our matrix over 3 lines. In this case, instead of the full
 `gap>`, a partial prompt `>` will be displayed until the user finishes
 the input with a semicolon:
 
-```source
+```gap
 gap> m:=[[ 1, 2, 3 ],
 >        [ 4, 5, 6 ],
 >        [ 7, 8, 9 ]];
@@ -183,7 +183,7 @@ gap> m:=[[ 1, 2, 3 ],
 
 You can use `Display` to pretty-print variables, including this matrix:
 
-```source
+```gap
 Display(m);
 ```
 
@@ -209,7 +209,7 @@ We will explain the differences in these outputs later.
 
 Here are some examples of calling other GAP functions:
 
-```source
+```gap
 Factorial(100);
 ```
 
@@ -221,7 +221,7 @@ Factorial(100);
 
 (the exact width of output will depend on your terminal settings),
 
-```source
+```gap
 Determinant(m);
 ```
 
@@ -231,7 +231,7 @@ Determinant(m);
 
 and
 
-```source
+```gap
 Factors(2^64-1);
 ```
 
@@ -245,7 +245,7 @@ used as arguments of other functions, for example, the
 all elements of the list which satisfy the function.
 `IsEvenInt`, unsurprisingly, checks if an integer is even!
 
-```source
+```gap
 Filtered( [2,9,6,3,4,5], IsEvenInt);
 ```
 
@@ -257,7 +257,7 @@ A useful time-saving feature of the GAP command-line interfaces is completion
 of identifiers when the Tab key is pressed. For example, type `Fib` and then
 press the Tab key to complete the input to `Fibonacci`:
 
-```source
+```gap
 Fibonacci(100);
 ```
 
@@ -281,7 +281,7 @@ for general use. Use them with extreme care!
 It is important to remember that GAP is case-sensitive. For example, the following
 input causes an error:
 
-```source
+```gap
 factorial(100);
 ```
 
@@ -340,7 +340,7 @@ own computer, you can set the help viewer to the default browser. If you are
 running GAP on a remote machine, this (probably) will not work. (see
 `?WriteGapIniFile` on how to make this setting permanent):
 
-```source
+```gap
 SetHelpViewer("browser");
 ```
 
@@ -358,13 +358,13 @@ This guide shows how permutations in GAP are written in cycle notation, and also
 shows common functions which are used with groups. Also, in some places two semi-colons
 are used at the end of a line. This stops GAP from showing the result of a computation.
 
-```source
+```gap
 a:=(1,2,3);;b:=(2,3,4);;
 ```
 
 Next, let `G` be a group generated by `a` and `b`:
 
-```source
+```gap
 G:=Group(a,b);
 ```
 
@@ -374,7 +374,7 @@ Group([ (1,2,3), (2,3,4) ])
 
 We may explore some properties of `G` and its generators:
 
-```source
+```gap
 Size(G); IsAbelian(G); StructureDescription(G); Order(a);
 ```
 
@@ -391,7 +391,7 @@ the entry from the Tutorial does not seem relevant, but the entry from the
 Reference manual is. It also explains the difference between using `AsSSortedList`
 and `AsList`. So, this is the list of elements of `G`:
 
-```source
+```gap
 AsList(G);
 ```
 
@@ -406,7 +406,7 @@ course, we may use the command line history to restore the last command, edit
 it and call again. But instead, we will use `last` which is a special variable
 holding the last result returned by GAP:
 
-```source
+```gap
 elts:=last;
 ```
 
@@ -418,7 +418,7 @@ elts:=last;
 This is a list. Lists in GAP are indexed from 1.
 The following commands are (hopefully!) self-explanatory:
 
-```source
+```gap
 gap> elts[1]; elts[3]; Length(elts);
 ```
 
@@ -439,14 +439,14 @@ gap> elts[1]; elts[3]; Length(elts);
 - Not required to contain objects of the same type
 
 - See more in [GAP Tutorial: Lists and Records](https://www.gap-system.org/Manuals/doc/tut/chap3.html)
-  
+
 
 ::::::::::::::::::::::::::::::::::::::::::::::::::
 
 Many functions in GAP refer to `Set`s. A set in GAP is just a list that happens to have
 no repetitions, no holes, and elements in increasing order. Here are some examples:
 
-```source
+```gap
 gap> IsSet([1,3,5]); IsSet([1,5,3]); IsSet([1,3,3]);
 ```
 
@@ -463,15 +463,15 @@ here.
 A `for` loop in GAP allows you to do something for every member of a collection.
 The general form of a `for` loop is:
 
-```source
+```gap
 for val in collection do
   <something with val>
 od;
 ```
 
 For example, to find the average order of our group `G` we can do.
 
-```source
+```gap
 s:=0;;
 for g in elts do
   s := s + Order(g);
@@ -487,7 +487,7 @@ Actually, we can just directly loop over the elements of `G` (in general GAP
 will let you loop over most types of object). We have to switch to using `Size`
 instead of `Length`, as groups don't have a length!
 
-```source
+```gap
 s:=0;;
 for g in G do
   s := s + Order(g);
@@ -502,7 +502,7 @@ s/Size(G);
 There are other ways of looping. For example, we can instead loop over a range of integers,
 and accept `elts` like an array:
 
-```source
+```gap
 s:=0;;
 for i in [ 1 .. Length(elts) ] do
   s := s + Order( elts[i] );
@@ -517,7 +517,7 @@ s/Length(elts);
 However, often there are more compact ways of doing things. Here is a very
 short way:
 
-```source
+```gap
 Sum( List( elts, Order ) ) / Length( elts );
 ```
 
@@ -551,7 +551,7 @@ and returns the value of the expression `e`. Here are some examples:
 
 - finding all elements of `G` with no fixed points:
 
-```source
+```gap
 Filtered( elts, g -> NrMovedPoints(g) = 4 );
 ```
 
@@ -561,7 +561,7 @@ Filtered( elts, g -> NrMovedPoints(g) = 4 );
 
 - finding a permutation in `G` that conjugates (1,2) to (2,3)
 
-```source
+```gap
 First( elts, g -> (1,2)^g = (2,3) );
 ```
 
@@ -571,7 +571,7 @@ First( elts, g -> (1,2)^g = (2,3) );
 
 Let's check this (remember that in GAP permutations are multiplied from left to right!):
 
-```source
+```gap
 (1,2,3)^-1*(1,2)*(1,2,3)=(2,3);
 ```
 
@@ -581,7 +581,7 @@ true
 
 - checking whether all elements of `G` move the point 1 to 2:
 
-```source
+```gap
 ForAll( elts, g -> 1^g <> 2 );
 ```
 
@@ -591,7 +591,7 @@ false
 
 - checking whether there is an element in `G` which moves exactly two points:
 
-```source
+```gap
 ForAny( elts, g -> NrMovedPoints(g) = 2 );
 ```
 
@@ -606,7 +606,7 @@ false
 - `Filtered( elts, g -> 2^g = 2 );`
 
 - `Filtered( elts, g -> (1,2)^g = (1,2) );`
-  
+
 
 ::::::::::::::::::::::::::::::::::::::::::::::::::