'expresson list' refers to the grammar term 'expression_list' in the subscription production.
value in the mapping that corresponds to that key. (The expression list is a
tuple except if it has exactly one item.)
-If the primary is a sequence, the expression (list) must evaluate to an integer
+If the primary is a sequence, the expression list must evaluate to an integer
or a slice (as discussed in the following section).
The formal syntax makes no special provision for negative indices in