[m-rev.] For review: add reference in "Promising purity" section

[Ralph Becket]

Estimated time taken: 0.1
Branches: main

doc/reference_manual.texi:
	Add a reference to the promise_purity scopes described in
	the `Goals' section of the reference manual, from the
	`Promising purity' section.

Index: reference_manual.texi
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RCS file: /home/mercury1/repository/mercury/doc/reference_manual.texi,v
retrieving revision 1.390
diff -u -r1.390 reference_manual.texi
--- reference_manual.texi	4 Apr 2007 04:38:49 -0000	1.390
+++ reference_manual.texi	25 May 2007 02:35:16 -0000
@@ -9139,12 +9139,16 @@
 Programmers should be very careful about mixing code that is promised
 pure with impure predicates or functions that may manipulate the
 same hidden state (for example, the impure predicates used to implement 
-a predicate that is promised pure); the @samp{promise pure} declaration
+a predicate that is promised pure); the @samp{promise_pure} declaration
 is supposed to promise that impure code cannot change the declarative
 semantics of pure code.  The module system can be used to minimize the
 possibility of making errors with such code, by keeping impure
 predicates or functions behind the interface where code is promised
 pure.
+
+Note that @code{promise_purity} pragmas are largely redundant due to 
+the @samp{promise_pure}, @samp{promise_semipure}, and @samp{promise_impure}
+casts described in @ref{Goals}.
 
 @node Impurity Example
 @section An example using impurity
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