A pairlist of the names of open graphics devices is stored in
.Devices. The name of the active device (see
dev.cur) is stored in .Device. Both are symbols
and so appear in the base namespace.
.Device is a length-one character vector.
.Devices is a pairlist of length-one character vectors.
The first entry is always "null device", and there are as many
entries as the maximal number of graphics devices which have been
simultaneously active. If a device has been removed, its entry will be
"" until the device number is reused.
Devices may add attributes to the character vector: for example
devices which write to a file may record its path in attribute
"filepath".