Add a notes section to the docs:

* Discuss representation error versus loss of significance.
* Document special values including qNaN, sNaN, +0, -0.
* Show the suprising display of non-normalized zero values.

Doc/lib/libdecimal.tex

@@ -828,6 +828,102 @@ The following table summarizes the hierarchy of signals:
 \end{verbatim}            
 
 
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\subsection{Floating Point Notes \label{decimal-notes}}
+
+The use of decimal floating point eliminates decimal representation error
+(making it possible to represent \constant{0.1} exactly); however, some
+operations can still incur round-off error when non-zero digits exceed the
+fixed precision.
+
+The effects of round-off error can be amplified by the addition or subtraction
+of nearly offsetting quantities resulting in loss of significance.  Knuth
+provides two instructive examples where rounded floating point arithmetic with
+insufficient precision causes the break down of the associative and
+distributive properties of addition:
+
+\begin{verbatim}
+# Examples from Seminumerical Algorithms, Section 4.2.2.
+>>> from decimal import *
+>>> getcontext().prec = 8
+
+>>> u, v, w = Decimal(11111113), Decimal(-11111111), Decimal('7.51111111')
+>>> (u + v) + w
+Decimal("9.5111111")
+>>> u + (v + w)
+Decimal("10")
+
+>>> u, v, w = Decimal(20000), Decimal(-6), Decimal('6.0000003')
+>>> (u*v) + (u*w)
+Decimal("0.01")
+>>> u * (v+w)
+Decimal("0.0060000")
+\end{verbatim}
+
+The \module{decimal} module makes it possible to restore the identities
+by expanding the precision sufficiently to avoid loss of significance:
+
+\begin{verbatim}
+>>> getcontext().prec = 20
+>>> u, v, w = Decimal(11111113), Decimal(-11111111), Decimal('7.51111111')
+>>> (u + v) + w
+Decimal("9.51111111")
+>>> u + (v + w)
+Decimal("9.51111111")
+>>> 
+>>> u, v, w = Decimal(20000), Decimal(-6), Decimal('6.0000003')
+>>> (u*v) + (u*w)
+Decimal("0.0060000")
+>>> u * (v+w)
+Decimal("0.0060000")
+\end{verbatim}
+
+
+The number system for the \module{decimal} module provides special
+values including \constant{NaN}, \constant{sNaN}, \constant{-Infinity},
+\constant{Infinity}, and two zeroes, \constant{+0} and \constant{-0}.
+
+Infinities can constructed directly with:  \code{Decimal('Infinity')}. Also,
+they can arise from dividing by zero when the \exception{DivisionByZero}
+signal is not trapped.  Likewise, when the \exception{Overflow} signal is not
+trapped, infinity can result from rounding beyond the limits of the largest
+representable number.
+
+The infinities are signed (affine) and can be used in arithmetic operations
+where they get treated as very large, indeterminate numbers.  For instance,
+adding a constant to infinity gives another infinite result.
+
+Some operations are indeterminate and return \constant{NaN} or when the
+\exception{InvalidOperation} signal is trapped, raise an exception.  For
+example, \code{0/0} returns \constant{NaN} which means ``not a number''.  This
+variety of \constant{NaN} is quiet and, once created, will flow through other
+computations always resulting in another \constant{NaN}.  This behavior can be
+useful for a series of computations that occasionally have missing inputs ---
+it allows the calculation to proceed while flagging specific results as
+invalid.     
+
+A variant is \constant{sNaN} which signals rather than remaining quiet
+after every operation.  This is a useful return value when an invalid
+result needs to interrupt a calculation for special handling.
+
+The signed zeros can result from calculations that underflow.
+They keep the sign that would have resulted if the calculation had
+been carried out to greater precision.  Since their magnitude is
+zero, the positive and negative zero are treated as equal and their
+sign is informational.
+
+In addition to the two signed zeros which are distinct, yet equal,
+there are various representations of zero with differing precisions,
+yet equivalent in value.  This takes a bit of getting used to.  For
+an eye accustomed to normalized floating point representations, it
+is not immediately obvious that the following calculation returns
+a value equal to zero:          
+
+\begin{verbatim}
+>>> 1 / Decimal('Infinity')
+Decimal("0E-1000000026")
+\end{verbatim}
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \subsection{Working with threads \label{decimal-threads}}
 
@@ -864,7 +960,7 @@ t2.start()
 t3.start()
  . . .
 \end{verbatim}
- 
+
 
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%