An atom is either an integer or an identifier.
A list is a left parenthesis, followed by zero or more S-expressions, followed by a right parenthesis.
An S-expression is an atom or a list.
NIL means ``false.'' NIL
is also the name for the empty list, and may be written as (
). In addition, NIL is unique in that it is at
once both an atom and a list (just learn this, don't try to make it
make sense).
T means ``true,'' but actually anything
that is not NIL can be used to mean ``true.''
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