The LaTeX package "
struktex" for drawing structograms break the "\boldsymbol" command, which is used for bold letters in math mode. A little investigation shows that the sub-package "struktxf" does some font magic to define the commands \nat, \integer, ... Fortunately this sub-package doesn't define anything if the command "\nat" is already defined.
To re-enable "
\boldsymbol" simply define the command "\nat" before using the package "struktex".
% Struktogramm
\def\nat{}
\usepackage{struktex}
