PEP 671: Clarify some points about the current options

pep-0671.rst

@@ -71,14 +71,14 @@ allows the expression to refer to other arguments.
 
 Multiple late-bound arguments are evaluated from left to right, and can refer
 to previously-defined values. Order is defined by the function, regardless of
-the order in which keyword arguments may be passed. Using names of other
-arguments is an error. It is implementation-defined whether this is a syntax
-error or a run-time error, and implementations are free to be more permissive,
-but this should not be relied upon.
+the order in which keyword arguments may be passed. Using names of later
+arguments should not be relied upon, and while this MAY work in some Python
+implementations, it should be considered dubious::
 
-    def spaminate(sausage=>eggs + 1, eggs=>sausage - 1): # Error
-    def selfref(spam=>spam): # Error
-    def frob(n=>len(items), items=[]): # Error
+    def prevref(word="foo", a=>len(word), b=>a//2): # Valid
+    def selfref(spam=>spam): # Highly likely to give an error
+    def spaminate(sausage=>eggs + 1, eggs=>sausage - 1): # Confusing, may fail
+    def frob(n=>len(items), items=[]): # May fail, may succeed
 
 
 Choice of spelling
@@ -87,15 +87,18 @@ Choice of spelling
 Our chief syntax proposal is ``name=>expression`` -- our two syntax proposals
 ... ahem. Amongst our potential syntaxes are::
 
+    # Preferred options: adorn the equals sign (approximate preference order)
     def bisect(a, hi=>len(a)):
     def bisect(a, hi=:len(a)):
     def bisect(a, hi:=len(a)):
     def bisect(a, hi?=len(a)):
     def bisect(a, hi!=len(a)):
     def bisect(a, hi=\len(a)):
-    def bisect(a, hi=`len(a)`):
     def bisect(a, hi=@len(a)):
+    # Less preferred option: adorn the variable name
     def bisect(a, @hi=len(a)):
+    # Less preferred option: adorn the expression
+    def bisect(a, hi=`len(a)`):
 
 Since default arguments behave largely the same whether they're early or late
 bound, the preferred syntax is very similar to the existing early-bind syntax.