gr <- GRanges("chr1:1-1000000:+")
gr2 <- GRanges(c("chr1:1-1000000:+", "chr2:1-1000000:+"))
gr3 <- GRanges(c("chr1:1-1000000:+", "chr1:1000000-2000000:+"))
gr4 <- GRanges(c("chr1:1-1000000:+", "chr1:1000000-2000000:-"))
gr5 <- GRanges(c("chr1:1-1000000:-"))