The inert function sync() operates analogously to
ggplot2::waiver() to indicate that an auxiliary aesthetic should be
synchronized to a standard aesthetic (when it is mapped from data).
is.sync() reports whether an object is of this class.
sync()is.sync(x)
sync() returns an empty list with the "sync" class; is.sync()
returns a logical value of whether x inherits this class.
An object to test