Typos, formatting.

source/Collection_Data_Types.rst

@@ -101,7 +101,7 @@ Exercise
 --------
 
 .. note::
-   ``sum`` is a function which return the sum of all the elements of a list.
+   ``sum`` is a function that returns the sum of all the elements of a list.
 
 Wihout using the Python shell, tell what are the effects of the following statements::
 
@@ -135,12 +135,12 @@ Here, we compute the mean of the values contained in the list ``x``, after havin
 Exercise
 --------
 
-Draw the representation in memory of the following expressions. ::
+Draw the representation in memory of the ``x`` and ``y`` variables when the following code is executed::
 
    x = [1, ['a', 'b', 'c'], 3, 4]
    y = x[1]
    y[2] = 'z'
-   # what is the value of x ?
+   # What is the value of x?
 
 .. figure:: _static/figs/list_2-1.png
    :width: 400px
@@ -152,7 +152,7 @@ Draw the representation in memory of the following expressions. ::
 
     .. image :: _static/figs/spacer.png
 
- When we execute *y = x[1]*, we create ``y`` wich reference the list ``['a', 'b', 'c']``.
+ When we execute *y = x[1]*, we create ``y`` which references the list ``['a', 'b', 'c']``.
  This list has 2 references on it: ``y`` and ``x[1]`` .
 
 
@@ -185,16 +185,22 @@ from the list l = [1, 2, 3, 4, 5, 6, 7, 8, 9] generate 2 lists l1 containing all
 Exercise
 --------
 
-generate a list containing all codons.
+.. note::
+    A codon is a triplet of nucleotides.
+    A nucleotide can be one of the four letters A, C, G, T
+
+Write a function that returns a list containing strings representing all possible codons.
+
+Write the pseudocode before proposing an implementation.
 
 pseudocode:
 """""""""""
 
 | *function all_codons()*
 |     *all_codons <- empty list*
-|     *let varying the first base*
-|     *for each first base let varying the second base*
-|     *for each combination first base, second base let varying the third base*
+|     *let vary the first base*
+|     *for each first base let vary the second base*
+|     *for each combination first base, second base let vary the third base*
 |     *add the concatenation base 1 base 2 base 3 to all_codons*
 |     *return all_codons*