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