Changed the rounding constant values to match their variable names.

This serves to made the context representation more useful (the names in
match the names out).

Removed the link to ReXX.  That document did not shed add anything to
information already in the spec.

Further tighten the public API to improve usability:

* Emax and Emin are set through Context().  Their defaults are set in the
  DefaultContext, so there is no need to expose DEFAULT_MAX_EXPONENT
  and DEFAULT_MIN_EXPONENT.

* The string functions isnan() and isinfinity() were only used internal to
  the module and were not among the test cases.  External use cases are
  served by the Decimal constructor.  So, made these private.

Lib/decimal.py

@@ -18,15 +18,10 @@ the General Decimal Arithmetic Specification:
 
     www2.hursley.ibm.com/decimal/decarith.html
 
-IEEE standard 854-1987:
+and IEEE standard 854-1987:
 
     www.cs.berkeley.edu/~ejr/projects/754/private/drafts/854-1987/dir.html
 
-and ANSI standard X3.274-1996:
-
-    www.rexxla.org/Standards/ansi.html
-
-
 Decimal floating point has finite precision with arbitrarily large bounds.
 
 The purpose of the module is to support arithmetic using familiar
@@ -128,19 +123,13 @@ __all__ = [
     'Inexact', 'InvalidContext', 'Rounded', 'Subnormal', 'Overflow',
     'Underflow',
 
-    # Module parameters
-    'DEFAULT_MAX_EXPONENT', 'DEFAULT_MIN_EXPONENT',
-
     # Constants for use in setting up contexts
     'ROUND_DOWN', 'ROUND_HALF_UP', 'ROUND_HALF_EVEN', 'ROUND_CEILING',
     'ROUND_FLOOR', 'ROUND_UP', 'ROUND_HALF_DOWN',
     'Signals',    # <-- Used for building trap/flag dictionaries
 
     # Functions for manipulating contexts
-    'setcontext', 'getcontext',
-
-    # Functions for working with decimals
-    'isinfinity', 'isnan',
+    'setcontext', 'getcontext'
 ]
 
 import threading
@@ -153,17 +142,17 @@ DEFAULT_MAX_EXPONENT = 999999999
 DEFAULT_MIN_EXPONENT = -999999999
 
 #Rounding
-ROUND_DOWN = 'down'
-ROUND_HALF_UP = 'half_up'
-ROUND_HALF_EVEN = 'half_even'
-ROUND_CEILING = 'ceiling'
-ROUND_FLOOR = 'floor'
-ROUND_UP = 'up'
-ROUND_HALF_DOWN = 'half_down'
+ROUND_DOWN = 'ROUND_DOWN'
+ROUND_HALF_UP = 'ROUND_HALF_UP'
+ROUND_HALF_EVEN = 'ROUND_HALF_EVEN'
+ROUND_CEILING = 'ROUND_CEILING'
+ROUND_FLOOR = 'ROUND_FLOOR'
+ROUND_UP = 'ROUND_UP'
+ROUND_HALF_DOWN = 'ROUND_HALF_DOWN'
 
 #Rounding decision (not part of the public API)
-NEVER_ROUND = 'never'    # Round in division (non-divmod), sqrt ONLY
-ALWAYS_ROUND = 'always'  # Every operation rounds at end.
+NEVER_ROUND = 'NEVER_ROUND'    # Round in division (non-divmod), sqrt ONLY
+ALWAYS_ROUND = 'ALWAYS_ROUND'  # Every operation rounds at end.
 
 #Errors
 
@@ -468,17 +457,17 @@ class Decimal(object):
         # String?
         # REs insist on real strings, so we can too.
         if isinstance(value, basestring):
-            if isinfinity(value):
+            if _isinfinity(value):
                 self._exp = 'F'
                 self._int = (0,)
-                sign = isinfinity(value)
+                sign = _isinfinity(value)
                 if sign == 1:
                     self._sign = 0
                 else:
                     self._sign = 1
                 return
-            if isnan(value):
-                sig, sign, diag = isnan(value)
+            if _isnan(value):
+                sig, sign, diag = _isnan(value)
                 if len(diag) > context.prec: #Diagnostic info too long
                     self._sign, self._int, self._exp = \
                                 context._raise_error(ConversionSyntax)
@@ -2127,8 +2116,8 @@ class Context(object):
     flags  - When an exception is caused, flags[exception] is incremented.
              (Whether or not the trap_enabler is set)
              Should be reset by user of Decimal instance.
-    Emin -   Minimum exponent (defaults to -999999999)
-    Emax -   Maximum exponent (defaults to 999999999)
+    Emin -   Minimum exponent
+    Emax -   Maximum exponent
     capitals -      If 1, 1*10^1 is printed as 1E+1.
                     If 0, printed as 1e1
                     (Defaults to 1)
@@ -2140,7 +2129,7 @@ class Context(object):
     def __init__(self, prec=None, rounding=None,
                  trap_enablers=None, flags=None,
                  _rounding_decision=None,
-                 Emin=DEFAULT_MIN_EXPONENT, Emax=DEFAULT_MAX_EXPONENT,
+                 Emin=None, Emax=None,
                  capitals=1, _clamp=0,
                  _ignored_flags=[]):
         if flags is None:
@@ -2927,15 +2916,15 @@ _infinity_map = {
     '-infinity' : -1
 }
 
-def isinfinity(num):
+def _isinfinity(num):
     """Determines whether a string or float is infinity.
 
-    +1 for positive infinity; 0 for finite ; +1 for positive infinity
+    +1 for negative infinity; 0 for finite ; +1 for positive infinity
     """
     num = str(num).lower()
     return _infinity_map.get(num, 0)
 
-def isnan(num):
+def _isnan(num):
     """Determines whether a string or float is NaN
 
     (1, sign, diagnostic info as string) => NaN
@@ -2978,6 +2967,8 @@ DefaultContext = Context(
         trap_enablers=dict.fromkeys(Signals, 0),
         flags=None,
         _rounding_decision=ALWAYS_ROUND,
+        Emax=DEFAULT_MAX_EXPONENT,
+        Emin=DEFAULT_MIN_EXPONENT
 )
 
 # Pre-made alternate contexts offered by the specification