Close bug 480337: Dict used before dicts explained. Added explanation

and examples of the dict() constructor.

Doc/tut/tut.tex

@@ -1843,8 +1843,6 @@ parenthesized.
 [12, 18]
 >>> [3*x for x in vec if x < 2]
 []
->>> [{x: x**2} for x in vec]
-[{2: 4}, {4: 16}, {6: 36}]
 >>> [[x,x**2] for x in vec]
 [[2, 4], [4, 16], [6, 36]]
 >>> [x, x**2 for x in vec]	# error - parens required for tuples
@@ -2023,6 +2021,17 @@ Here is a small example using a dictionary:
 1
 \end{verbatim}
 
+The \function{dict()} contructor builds dictionaries directly from
+lists of key-value pairs stored as tuples.  When the pairs form a
+pattern, list comprehensions can compactly specify the key-value list.
+
+\begin{verbatim}
+>>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
+{'sape': 4139, 'jack': 4098, 'guido': 4127}
+>>> dict([(x, x**2) for x in vec])     # use a list comprehension
+{2: 4, 4: 16, 6: 36}
+\end{verbatim}
+
 
 \section{Looping Techniques \label{loopidioms}}