diff --git a/latex/paquets.sty b/latex/paquets.sty
index 1ecdc37..7d20f2c 100644
--- a/latex/paquets.sty
+++ b/latex/paquets.sty
@@ -20,6 +20,7 @@
 \RequirePackage{mathtools}
 \RequirePackage{xspace}
 \RequirePackage{mdwlist}
+\RequirePackage{xparse}
 
 \defaultfontfeatures{Ligatures=TeX}
 \setmainfont[SlantedFont=Gentium Basic, SlantedFeatures={FakeSlant},SmallCapsFont=Gentium Plus, SmallCapsFeatures={Letters=SmallCaps}]{Gentium Basic}
diff --git a/latex/yssymbols.sty b/latex/yssymbols.sty
index aa494a7..178be40 100644
--- a/latex/yssymbols.sty
+++ b/latex/yssymbols.sty
@@ -61,8 +61,8 @@
 \def\lang#1{\mathscr L\left(#1\right)}
 \let\peuttrans\rightsquigarrow
 
-\def\enspart#1{\mathscr P\left(#1\right)}
-\def\enspartf#1{{\mathscr P}_{\mathrm{fini}}\left(#1\right)}
+\DeclareDocumentCommand\enspart{om}{\mathscr P\IfNoValueTF{#1}{}{_{#1}}{\left(#2\right)}}
+\DeclareDocumentCommand\enspartf{m}{\enspart[\mathrm{fini}]{#1}}
 
 \def\ie{\textsl{ie.}\xspace}
 \newcommand{\tvers}[1]{\xrightarrow[\ \: #1 \:\ ]{}}