strip_unapproved is an internal function which
essentially 'deletes' layers from the current ternary plot
in the event that such layers are not one of the approved
layers. Refer to approved_geometries for the current
list of approved geometries.
Usage
strip_unapproved(layers)
Arguments
layers
list of the layers to strip unnaproved
layers from.
Value
strip_unapproved returns a list of approved layers
(may be empty if none are approved).