Indented expressions.

iexp, iexpr.

sexp means S-expressions.

All code is made up of trees, that is mathematical trees, which is the same as
hiercial structures. The next question must then be, how do one represent these
trees in the most efficient way. Mathematics have had one sollution to this,
which lisp coders have adopted and made into their sexp, which are simply parens
enclosing the elements of a list. This type of list is very simple to parse, and
generate. Consider the following code:
(trunk (branch (leaf leaf) (branch (leaf leaf))))

Because this isn't very easy to read, I'll format it a little for you:
(trunk (branch (leaf
                leaf)
       (branch (leaf
                leaf))))

This isn't the standard lisp formating scheme, but it is very easy to read, and
see the tree structure.

Notice all the end parens, many people think those look very ugly, but I and all
lisp coders don't even realize they are there. They become a natural part of the
code, and I don't have to read them to understand the code. What if I removed
them then?

trunk branch leaf
             leaf
      branch leaf
             leaf

I can still understand the structure of this, and so can most people (even
non-lispers). This is what I call i-expressions, or iexp.

If I give you the task to draw a heirarcy many of you would draw something like
this:

          _______
      ___/ trunk \__
 ____/branch | branch\____
/leaf | leaf | leaf | leaf\
---------------------------

One can prove this to oneself by look at the following url:
http://images.google.com/images?q=hierarchy