+=item C<call_sub $name, @args>
+
+Calls the sub C<$name> on C<$self> with the arguments C<@args> and
+returns its result. This is meant for situations in which the sub's
+name is a composite, e.g.
+
+ my $chart_id = $buchungsgruppe->call_sub(($is_sales ? "income" : "expense") . "_accno_id_${taxzone_id}");
+
+=item C<call_sub_if $name, $check, @args>
+
+Calls the sub C<$name> on C<$self> with the arguments C<@args> if
+C<$check> is trueish. If C<$check> is a code reference then it will be
+called with C<$self> as the only argument and its result determines
+whether or not C<$name> is called.
+
+Returns the sub's result if the check is positive and C<$self>
+otherwise.
+