Lisp is a practical, expression-oriented, interactive programming language which uses linked lists as one of its major data structures. A Lisp list is written with its elements separated by whitespace, and surrounded by parentheses. Lisp source code is itself comprised of lists. The language has many unique features that make it excellent to study programming constructs and data structures. Many regard Lisp as an extremely natural language to code complex symbolic reasoning programs. Lisp is popular in the fields of artificial intelligence and symbolic algebra.