(int) ClearCell(undoable, layer_list)
This function will clear the content of the present mode (electrical
or physical) part of the current cell. If the first argument is
nonzero, the deletions will be added to the internal undo list,
otherwise not. The latter is more efficient, though this makes the
deletions irreversible. The second argument, if null or empty,
indicates that all objects on all layers will be deleted, including
subcells. Otherwise this can be set to a string containing a
space-separated list of layer names, following an optional special
character `!' or `^' which must be the first
character in the string if used. If the special character does not
appear, the deletions apply only to the layers listed. If the special
character appears, the deletions apply only to the layers not
listed. Recall that the internal name for the layer that contains
subcells ls ``$$'', thus for example using ``! $$''
would delete all geometry but retain the subcells.
The return value is the number of objects deleted.