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usepackage[ntheorem,framemethod=tikz]{mdframed}
usepackage[amsmath]{ntheorem}
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documentclass[12pt,fleqn,a4paper]{article}
usepackage{amsmath}
usepackage[a4paper,textwidth=16cm,vscale=0.75,vcentering]{geometry}
usepackage{framed}
usepackage[amsmath,framed]{ntheorem}
usepackage{parskip}

% Modified theorem header
makeatletter
    deftheorem@checkbold{}
makeatother

makeatletter
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makeatother

% Remove extra vertical skip at the end of frames
letsvendmdframedendmdframed
makeatletter
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% Definition
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begin{document}
    begin{definition}[Basic set operations.]
        Donec vehicula augue euneque.

        bigskip
        {textbf{Set equality:}}\
        Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit,
        begin{equation}
            A + B = C.
        end{equation}
        Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna.

        bigskip
        {textbf{Set union:}}\
        Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit,
        begin{equation}
            A + B = C.
        end{equation}
        Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna.

        bigskip
        {textbf{Set intersection:}}\
        Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit,
        begin{equation}
            A + B = C.
        end{equation}
        Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna.

        bigskip
        {textbf{Set difference:}}\
        Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit,
        begin{equation}
            A + B = C.
        end{equation}
        Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna.

        bigskip
        {textbf{Set complement:}}\
        Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit,
        begin{equation}
            A + B = C.
        end{equation}
        Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna.

        bigskip
        {textbf{Cartesian product:}}\
        Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit,
        begin{equation}
            A + B = C.
        end{equation}
    end{definition}
end{document}