[racket] boolean in PLAI

We always spelled it out as `true` and `false` (and, yes, you have to
check for those before parsing other identifiers).

I wouldn't mind allowing booleans within S-expression, though, if you'd
like me to add it.

At Thu, 16 Jan 2014 15:26:11 -0500, Marco Morazan wrote:
> Dear All,
> 
> In plai-typed, an S-expression cannot be a boolean, correct? How can I
> parse a boolean? The idea here is to construct a boolean expression when a
> boolean is encountered in the input/
> 
> -- 
> 
> Cheers,
> 
> Marco
> 
> Have a´¨)
> ¸.·´¸.·*´¨) ¸.·*¨)
> (¸.·´ (¸.·´ * wonderful day! :)
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