\usepackage{amsopn}
\fi
-\input{mjo-common} % for \of, at least
+\input{mjo-common} % for \of, and \binopmany
+
+
+% The direct sum of two things.
+\newcommand*{\directsum}[2]{ {#1}\oplus{#2} }
+
+% The direct sum of three things.
+\newcommand*{\directsumthree}[3]{ \directsum{#1}{\directsum{#2}{#3}} }
+
+% The (indexed) direct sum of many things.
+\newcommand*{\directsummany}[3]{ \binopmany{\bigoplus}{#1}{#2}{#3} }
% The (sub)algebra generated by its argument, a subset of some ambient