Hi,
In my project, there is an env-03.tex file that contains this :

""""""""""""""""""""""""""""""""""""""""""""""""
\startenvironment env-03

\defineenumeration
  [def]
  [text={Définition},
   headstyle={\bf\feature[+][f:smallcaps]},
   width=fit,
   alternative=left,
   stopper={.}]

\stopenvironment

"""""""""""""""""""""""""""""""""""""""""""""""""

If I compile, I get this :

open source     > 4 > 119 > env-03.tex

tex error       > tex error on line 10 in file env-03.tex: ! You can't use
a prefix with `\begingroup'

<to be read again>
\begingroup
\strc_enumerations_command #1->\begingroup
                                           \strc_constructions_initialize
{#...
<to be read again>
\20>>1
\dostepwiserecurse ...on \recursedepth \endcsname
                                                  ##1##2{#4}\global
\expanda...
<inserted text> ...numerationsub {\v!sub \currentenumerationsub }}
                                                  \else \let
\currentenumera...
\define_enumeration ...he \everydefineenumeration
                                                  \let \currentenumeration
\...
...
l.10


 1     \startenvironment env-03
 2
 3     \defineenumeration
 4       [def]
 5       [text={Définition},
 6        headstyle={\bf\feature[+][f:smallcaps]},
 7        width=fit,
 8        alternative=left,
 9        stopper={.}]
10 >>
11     \stopenvironment
12

Thanks,
Fabrice