improved backslash d

lecture.cls

@@ -138,8 +138,8 @@
 }%
 
 % integral d sign
-\makeatletter \renewcommand\d[1]{\ensuremath{%
-  \;\mathrm{d}#1\@ifnextchar\d{\!}{}}}
+\makeatletter \renewcommand\d[2][]{\ensuremath{%
+		\,\mathrm{d}^{#1}#2\@ifnextchar^{}{\@ifnextchar\d{}{\,}}}}
 \makeatother
 
 % remove page before chapters