https://doi.org/10.1351/goldbook.A00247
A @S06078@ occurring at position 1/ of an @A00245@ system, the double bond being between positions 2/ and 3/. The incoming group may be attached to the same atom 1/ as the @L03493@, or the incoming group becomes attached at the relative position 3/, with movement of the double bond from 2/3 to 1/2. For example:
A00247.png
(written as a @T06446@).