Sources which lie close to each other may generate event-extraction regions which overlap. It is obviously undesirable to include in a subset of events associated with one source events which are also associated with another. region provides a mechanism for avoiding this overlap. If the user selects shrinkconfused=`yes', and provided that the source region shape is circular not elliptical (true if radiusstyle is not equal to `contour', or if radiusstyle=`contour' but nosrcellipse=`yes'), the task will reduce the radii of pairs of overlapping source regions until they no longer overlap. Each source of the pair suffers the same fractional reduction.
Elliptical regions are not at present shrunk because the complexity of the geometry required does not seem to be compensated by the advantage gained.
Note that, in general, a source has a non-zero chance of being contaminated by events from every other source in the field of view (FOV). It is only possible to correct for this cross-contamination on a statistical basis, and even this is difficult to do without introducing systematic errors greater than the contamination one is attempting to remove. The shrinkage of extraction regions performed by region must be considered only a very coarse first step towards such correction.
Note that source-region cutouts from background regions are not shrunk.
