A recursive definition is a definition in which (1) certain things are specified as belonging to the category being defined, and (2) a rule or rules are given for building new things in the category from other things already known to be in the category.
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.''
also the name for the empty list, and may be written as
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.''