Add interval; i.e., add all integers from a to b to set.
If b<a, do nothing.
Keep list in sorted order (by left range value).
If overlap, combine ranges. For example,
If this is {1..5, 10..20}, adding 6..7 yields
{1..5, 6..7, 10..20}. Adding 4..8 yields {1..8, 10..20}.
Add interval; i.e., add all integers from a to b to set. If b<a, do nothing. Keep list in sorted order (by left range value). If overlap, combine ranges. For example, If this is {1..5, 10..20}, adding 6..7 yields {1..5, 6..7, 10..20}. Adding 4..8 yields {1..8, 10..20}.