A few useful sequence functions

[ghislainfourny]

The following functions would be very useful because they are very commonly used:

empty((2, 3)) -> false
exists((2, 3)) -> true
tail((2, 3, 4)) -> (3, 4)
head((2, 3)) -> 2

as well as

insert-before#3
remove#2
reverse#1
subsequence#2
subsequence#3,

The first three can be optimized, because only the first value (if here) needs to be accessed, and the rest ignored.