1. REBOL
Lists

2. REBOL does it differently
Other languages:
Lists (and similar sequences) are singly-linked
head returns the first element of the series
tail returns the remainder of the series, minus the head
REBOL:
Series are doubly-linked, hence traversable in either direction
Any reference to a series is a reference to some position in the series
head returns the beginning of the series
tail returns the end of the series

3. Traversing a sequence head returns the beginning of the sequence
tail returns the end of the sequence
next returns the next position in the sequence
If already at the tail, returns the tail
back returns the previous position in the sequence
If already at the head, returns the head
head? tests if a position is the beginning of a sequence
tail? tests if a position is the end of a sequence

4. Example traversals >> str: "REBOL" == "REBOL"
>> mid: next next str == "BOL"
>> head? mid == false
>> head mid == "REBOL"
>> end: tail str == ""
>> back end == "L"

5. Getting elements from a sequence
>> str: "REBOL" == "REBOL"
>> first str == #"R"
>> second str == #"E"
>> last str == #"L"
>> pick str 4 == #"O"
>> pick str 8 == none

6. Indexing into a sequence
>> str: "REBOL" == "REBOL"
>> index? str == 1
>> mid: at str 3 == "BOL"
>> at mid 2 == "OL”
>> at mid -1 == "EBOL"
>> index? at mid -1 == 2

7. The End