These are methods for extracting and replacing subsets
for the class "solist".
The argument x should be an object of class "solist"
representing a list of two-dimensional spatial objects.
See solist.
For the subset method, the subset index i can be either
a vector index (specifying some elements of the list)
or a spatial window (specifying a spatial sub-region).
For the replacement method,
i must be a vector index: the designated elements will be
replaced.