Thank, Wolfgang! Both methods work, but both fail wit= h the following example:

\pushmacro\unexpanded =C2= =A0
\let\unexpanded\normalunexpanded
\input xstring
\popmacro\unex= panded

\starttext
\StrGobbleLeft{bcdef}{4}
\stoptext

The log says:

tex error = > tex error on line 6 in file ./tests.tex: ! This can't happen (pr= efixed command)

<argument&g= t; \unexpanded {

f= }

\xs_execfirst = #1#2->#1

However, some others work.

El mar., 7 de jul. de 2020= a la(s) 15:20, Wolfgang Schuster (wolfgang.schuster.lists@gmail.com) escribi=C3=B3:

Phelype Oleinik schrieb = am 07.07.2020 um 20:53:
> Hi Jairo,
>
> ConTeXt's \unexpanded is e-TeX's \protected, so the error boil= s down to
> something like: \edef\x{\unexpanded{#}}, which works in Plain but not = in
> ConTeXt.
>
> Usually packages that claim to be generic have to cope with that.
> xstring would need to do (in a ConTeXt-specific loader):
>=C2=A0 =C2=A0 =C2=A0 \let\xstring@unexpanded\normalunexpanded
> and
>=C2=A0 =C2=A0 =C2=A0 \let\xstring@unexpanded\unexpanded
> in Plain and LaTeX loaders, then then use \xstring@unexpanded in the > code.=C2=A0 But `xstring` doesn't claim (that I found, at least) t= o be
> compatible with ConTeXt, so it's understandable: though if it is > compatible with Plain, it wouldn't be hard to adapt to ConTeXt. >
> That said, you can load `xstring` if you do:
>=C2=A0 =C2=A0 =C2=A0 =C2=A0\let\unexpanded\normalunexpanded
>=C2=A0 =C2=A0 =C2=A0 =C2=A0\input xstring.tex
>=C2=A0 =C2=A0 =C2=A0 =C2=A0\let\unexpanded\protected
> but the internals of the package will probably be comprimised by the > misuse of (ConTeXt) \unexpanded.

Bad idea because ConTeXt also changes the meaning of \protected (you can use \normalprotected to get the primitve).

A better version is

\pushmacro\unexpanded
\let\unexpanded\normalunexpanded
\input xstring
\popmacro\unexpanded

Wolfgang
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