{{subst:subst}}#ifexpr:{{{n}}}>1 |{{subst:subst}}exp/sum |n={{subst:#expr:{{{n}}}-1}} |x={{{x}}} |sum={{{sum}}}+{{subst:exp/term1|n={{{n}}}|x={{{x}}}}} }} |{{{sum}}}}}

Substituting this template produces one or more new substitution template calls, allowing recursive change of a page, see m:Help:Recursive conversion of wikitext.