Hi Wolfgang,

Many thanks for your attention and the link you sent.
I looked up the chapter on references of the main manuel written by Hans, but unfortunately I could not find a solution, or could not adapt what is said there. 
It can be that I have to change my approach of defining my « exercises », for instance by defining each exercise as a section or subsection? 
Up to now I have two ways of defining an « exercise »: 
	• one is with \defineenumeration[Exo] (when using mkii), 
	• the other one is with \defineannotation[Exo] and your annotation module when using mkiv.

In neither case the command
	\setupreferencing[prefix=+]
or
	\setupannotation[Exo][prefix=+]
seems to generate local tags. 

Just in case, below is a minimal file, which shows how I use the annotation module.

Best regards: OK
%%%%%%%%%% begin
\setupinteraction[state=start]

\usemodule[annotation] % Wolfgang's module

\define[2]\ExoSetup
	{\resetnumber[formula]
	\setupframedtext[
		width=\textwidth,
		frame=off,
		leftframe=on]
	 \textrule{#1}%
	 \startframedtext
	 #2%
	 \stopframedtext}
	 
\defineannotation[Exo]
	[alternative=command,
	command=\ExoSetup,
	text=Exercise ,
	number=yes,
	headcolor=darkred,
	]

% \setupannotation[Exo][prefix=+]
\setupreferencing[prefix=+]

\starttext

\startExo[exo:CharacTwo] Prove that if $ab=ba$ and $2ab=0$ then
\placeformula[eq:1]
\startformula
(a+b)^2 = a^2 + b^2.
\stopformula
Using the identity (\in[eq:1]) prove that $(a=b)^{2n} = a^{2n} + b^{2n}$.
\stopExo

\page

\startExo[exo:Moivre]{Moivre's formula} Prove that for any integer $n$
\placeformula[eq:2] % I would like the reference to be [eq:1]
\startformula
(\cos(x) + {\rm i}\sin(x))^n = \cos(nx) + {\rm i} \sin(nx).
\stopformula
Using the identity (\in[eq:2]) % I would like here to say [eq:1]
 compute $\cos{3x}$.
\stopExo

\stoptext
%%%%%%%%%% end

On 28 nov. 2011, at 00:10, Wolfgang Schuster wrote:

> 
> Am 27.11.2011 um 23:27 schrieb Otared Kavian:
> 
>> Dear all,
>> 
>> Is it possible to have a sort of « local references for formulas » or else to clear previous references to formulas at a certain point?
>> In a chapter where a certain number of exercices and formulas are to be numbered, assume I have the following minimal example
>> 
>> %%%%%%%%% begin
>> \starttext
>> 
>> Exercice 1. Prove that if $ab=ba$ and $2ab=0$ then
>> \placeformula[eq:1]
>> \startformula
>> (a+b)^2 = a^2 + b^2.
>> \stopformula
>> Using the identity (\in[eq:1]) prove that $(a=b)^{2n} = a^{2n} + b^{2n}$.
>> 
>> \blank[big]
>> \hrule
>> \blank[big]
>> 
>> Exercice 2. Prove that for any integer $n$
>> \resetnumber[formula]
>> %\reset[formulas][references]
>> \placeformula[eq:2] % I would like the reference to be [eq:1]
>> \startformula
>> (\cos(x) + {\rm i}\sin(x))^n = \cos(nx) + {\rm i} \sin(nx).
>> \stopformula
>> Using the identity (\in[eq:2]) % I would like here to say [eq:1]
>> compute $\cos{3x}$.
>> 
>> \stoptext
>> %%%%%%%%% end
>> 
>> I would like to be able to have the equations numbered within each exercise (which is in fact defined through an enumeration environment), and be able to have the reference to each formula « local » within that exercise.
>> The resaon is that the execises and problems may have several equations which are referenced to with [eq:1] through [eq:10] (say…), and have been prepared and given previously to students on independent sheets. When I want to give out a booklet of all the exercises, then it would be handy to have such a feature.
>> 
>> I understand that I am not being very clear… Hope someone would understand what I mean and need :-)
> 
> Look at page 168: http://pragma-ade.com/general/manuals/cont-enp.pdf
> 
> Wolfgang
> ___________________________________________________________________________________
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> 
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%%%%%%%%%%%%%%%%%%
Otared Kavian
Département de Mathématiques
Université de Versailles Saint-Quentin
Bâtiment Fermat
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78035 Versailles cedex

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