r <- raster(ncol=100, nrow=100)
r1 <- crop(r, extent(-10, 11, -10, 11))
r2 <- crop(r, extent(0, 20, 0, 20))
r3 <- crop(r, extent(9, 30, 9, 30))
r1[] <- 1:ncell(r1)
r2[] <- 1:ncell(r2)
r3[] <- 1:ncell(r3)
m = mosaic(r1, r2, r3, fun=mean)
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