In some parts of guile, scm_t_bits and SCM variables are passed to
     functions as void* arguments.  Together with the requirement above,
     this requires a one-to-one correspondence between the size of a
     void* and the sizes of scm_t_bits and SCM variables.

   - numbers are encoded using two's complement.
     The implementation of the bitwise Scheme-level operations is based on
     this assumption.  */