Expressions containing mathematical prefixes are treated as numbers

[apophizzz]

When an expression including '+' or '-' is given, JScheme is expected to treat it as an undefined variable (assumed that is has not been defined before). This behavior conforms to the Racket specification. So what we would expect JScheme to do is something like this:

>> 1-2
### ERROR: undefined variable 1-2
>> 1+2
### ERROR: undefined variable 1+2

However, what JScheme currently does is interpreting expressions containing mathematical prefixes as numbers, thus returning unexpected results like this:

>> 1-2
=> 92
>> 1+2
=> 92