An Introduction to LISP
           ✔ How to interact with LISP:
           sdcc5.ucsd.edu% cl
           Allegro CL 4.1 [SPARC; R1] (4/9/93 17:00)
           Copyright (C) 1985-1992 Franz Inc., Berkeley CA, USA.
           ;; Optimization settings; safety 1, space 1, speed 1, debug 2
           ;; For a complete description of all compiler switches
           ;; evaluate (EXPLAIN-COMPILER-SETTINGS).
           USER(1):
           ✔ cl stands for Common LISP, now the standard dialect of LISP
           ✔ (Note the semicolons ; In LISP programs, comments start with a
              semicolon and go to the end of the line)
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           Interacting with LISP
           ✔ LISP is an interactive language
              ✗  (Common LISP also has a good compiler, and can do offline
                 computations)
           ✔ When you invoke LISP, the lisp interpreter enters a
              read - eval - print loop
           ✔ You type an expression in response to the prompt, and ...
              ✗  LISP reads in the expression you typed
              ✗  LISP evaluates it
              ✗  LISP prints the result of the evaluation, and prompts for the
                 next input
           ✔ (Note: in fact, read, eval, print are names of builtin LISP
              functions...)
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           Interacting with LISP
           ✔ What is the result of LISP’s evaluation process?
           ✔ It depends on the expression that is being evaluated
           ✔ Expressions are of two types:
              ✗   atoms
              ✗   lists
           ✔ Atoms are of two types:
              ✗   constants:
                   • numbers: 27, 3.14, 5/8, 1.0e26, etc.
                   • strings: “A string”, “another one”, etc.
                   • characters: #\a, #\A, #\b, etc.
              ✗   symbols: foobar, FooBaR, hyphenated-symbol, +, etc.
           ✔ A list is represented as
               (
                 < zero or more atoms or lists >
               )
              ✗   examples of lists:
               ()
               (item1 item2 item3)
               (foo (bar baz))
               (+ 3 4)
               (() (()()))
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          Expression types in LISP
                                        symbol
                              atom                    number
                                         constant     string
                                                      character
             s-expressions
                                       the empty list
                              list
                                       cons
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