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CheckMayLoad() method is used to determine whether or not a principal can load a specified URI.
If the originating principal is a
NullPrincipal, this test returns
false, denying the load.
If that test is
false, then some fairly involved special-casing takes effect to handle the same origin policy for
In general, the load is allowed if both the origin and target are
file: URIs, the target isn't a directory, and the target is either in a subdirectory of the origin (when the origin is a directory, which is rare for XBL) or is in a subdirectory of the origin's parent (when the origin is a file).