Not such an option here!
The only reverse engineering tool for sequence diagrams that I know is UMLgraph . For this you need a license, but, as I understand it, it is free from time. It is designed for Eclipse and can be installed quite easily. And reverse engineering class diagrams is not licensed. I checked them - class diagrams really do this reverse engineering.
Also, take a serious look at Papyrus. It is powerful and no worse than the best proprietary tools.
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