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<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://vi.symbolab.com/solver/step-by-step/%5Cint_%7B0%7D%5E%7B1%7D%5Cint_%7B0%7D%5E%7B1%7Dxy%5Ccdot+%5C%3Adx%5C%3Ady>