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$\{ helloworld \}$ yields {helloworld}
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\begin{cases}
c(a_i) = a_{i+1} & \text{for }1\le i<l\\
c(a_l) = a_1
\end{cases}
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To get curly braces in the output, you must use the escaped versions, "\{", and "\}".
For example:
Normal text \{Helper Text\}