Specifies that a double bond to 'X' must exist. Equivalent to writing '~X(bond=double)'. 'X' may be either an element symbol, an id for another type specifier, or a list containing one or both of these to allow more flexible specifications. If used inside the bracketed part of a ring description this only indicates that the atom/type should be present within the cycle - the connection to the target atom is unimportant. If used in a chain keyword the connection type is honoured.

For example: