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The following datatypes are immediate values; that is, they fit entirely
within an SCM value. The SCM_IMP and SCM_NIMP
macros will distinguish these from non-immediates; see Immediates vs Non-immediates for an explanation of the distinction.
Note that the type predicates for immediate values work correctly on any
SCM value; you do not need to call SCM_IMP first, to
establish that a value is immediate.