let str = "1234567890"
str[1..]  				// 234567890
str[..5]   				// 123456
str[1..4]  				// 2345
str[2..-2]				// 3456789
str[-4..-2]				// 789

str[5..4] str[-2..-4] str[4..-7] 	// nil (start larger than end)
st[11..]  str[..12]   str[..-12] 	// nil (out of range)
str[-1..2]               // nil (meaningless)

pod 'BetterSwift/String'