Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. SKI combinator calculus is a computational system that may be perceived as a reduced version of untyped Lambda calculus. All operations in Lambda calculus are expressed in SKI as binary trees whose leaves are one of the three symbols S, K, and I (called combinators). In fact, the symbol I is added only for convenience, and just the other two suffice for all of the purposes of the SKI system. Although the most formal representation of the objects in this system requires binary trees, they are usually represented, for typesettability, as parenthesized expressions, either with all the subtrees parenthesized, or only the right-side children subtrees parenthesized. So, the tree whose left subtree is the tree KS and whose right subtree is the tree SK is usually typed as ((KS)(SK)), or more simply as KS(SK), instead of being fully drawn as a tree (as formality and readability would require).