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### How to implement a minimal scheme?

Potentially need an AST-like data structure representing the parsed expression.
This is the s-expression. Well represented as a pair with value types:

    null
    pair
    boolean (true/false)
    number
    identifier/symbol

One way to accomplish the above would be to use a pair datatype in the host
language. Another is to try to use tuples or lists; this will result in faster
execution but is much harder?

'value' and other high level functions will take an s-expr and return an
s-expr. eval wraps this with an AST parse at the begining and a pretty print at
the end.

'meaning' takes an s-expr object and a context/environment and computes the
result s-expr. it recursively looks at each node in the s-expr, decides what
action needs to be taken on it, and executes the action. possible actions (aka,
parsed types) are:

    const (atomic values)
    lambda
    quote (s-expr "one level down")
    identifier (a symbol to be looked up in the context)
    cond (conditional expression)

The specific list of "special"/"built-in"/"reserved" identifiers is:

    lambda
    quote
    cond
    else
    cons
    car
    cdr
    null?
    eq?
    atom?
    zero?
    number?

Things which an be applied are lambdas and the above built-ins.

When applying a lambda, need to bind values to local context. This requires
an immutable or copied lookup table.