From: A rule-based algorithm for automatic bond type perception

Return number | 0 | 1 | 2 | 4 | 5 |
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Aii doesn't have a double bond
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Bond types are 5 and 6 | |||||

A1 has more than 1 bonds
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A3 has more than 1 bonds
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$\begin{array}{l}O12=4\\ O23>4\end{array}$ | |||||

$\begin{array}{l}O12>4\\ O23=4\end{array}$ | |||||

A2 has three bonds
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Others | others |