Conditional expressions, composition and recursion are the basic operations used in LISP to define functions on list structures. Any computable function of arbitrarily complex list structures may be described using these operations, but certain simple transformations of linear lists (strings) are awkward to define in this notation. Such transformations may be characterized (and caricaturized) by the following instructions for a transformation: "Take that substring there, and that other one starting with "Black", which has the substring mentioned third as the first; then inserts the second substring mentioned; omit the first and leave the unmentioned parts of the original string unchanged."
