#### Sequences

A sequence (abbreviated to seq) is a group of adjacent parenthesized elements. For example,
(a)(b)(c)
...is a seq of 3 elements--a, b, and c.
Sequences are data structures that merge the properties of both lists and tuples with the exception that a seq cannot be empty. Therefore, an "empty" seq is considered a special case scenario that must be handled separately in C++.

    #define SEQ (x)(y)(z)
    #define REVERSE(s, state, elem) (elem) state
       // append to head                  ^

    BOOST_PP_SEQ_FOLD_LEFT(REVERSE, BOOST_PP_EMPTY, SEQ)()
       //                           #1                  #2
       // 1) placeholder for "empty" seq
       // 2) remove placeholder

    #define SEQ_B (1)(2)(3)
    #define INC(s, state, elem) state (BOOST_PP_INC(elem))
       // append to tail             ^

    BOOST_PP_SEQ_FOLD_RIGHT(INC, BOOST_PP_SEQ_NIL, SEQ)
       //                        ^
       // special placeholder that will be "eaten"
       // by appending to the tail

Sequences are extremely efficient. Element access speed approaches random access--even with seqs of up to 256 elements. This is because element access (among other things) is implemented iteratively rather than recursively. Therefore, elements can be accessed at extremely high indices even on preprocessors with low maximum expansion depths.
Elements of a seq can be extracted with BOOST_PP_SEQ_ELEM.

#### Primitives

- BOOST_PP_SEQ_ELEM