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The immediate values that are neither small integers, characters, nor booleans are all unique values -- that is, datatypes with only one instance.
This is sort of a weirdly literal way to take things, but the standard read-eval-print loop prints nothing when the expression returns this value, so it's not a bad idea to return this when you can't think of anything else helpful.
For example, when you write a C function that is callable from Scheme
and which takes optional arguments, the interpreter passes
SCM_UNDEFINED for any arguments you did not receive.
We also use this to mark unbound variables.
SCM_UNDEFINED. Apply this to a symbol's value to see if it has a binding as a global variable.
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