``Marks'' are observations attached to each point of a point pattern.
For example the longleaf dataset
contains the locations of trees, each tree being marked by its diameter;
the amacrine dataset gives the locations of cells
of two types (on/off) and the type of cell may be regarded as a mark attached
to the location of the cell.
This function tests whether the point pattern X
contains or involves marked points, and that the
marks are a factor.
It is a method for the generic function is.multitype.
For example, the amacrine
dataset is multitype (there are two types of cells, on and off),
but the longleaf dataset is not multitype
(the marks are real numbers).
The argument na.action determines what action will be taken
if the point pattern has a vector of marks but some or all of the
marks are NA. Options are "fatal" to cause a fatal
error; "warn" to issue a warning and then return TRUE;
and "ignore" to take no action except returning TRUE.