In VRR
there may be an arbitrary depth of scheme/C switches on stack.
GUILE Scheme uses exceptions for error signalization. Both
exceptions and failed transactions contain some form of long jump. It
is necessary to ensure that a failed transaction does not jump over some
scheme sections on stack (and vice versa). It is accomplished by
wrapping each C code executed from Scheme code by an anonymous
transaction; and each Scheme code executed from the C code by the function
scm_call_with_dynamic_root() which disallows any indirect
returns from the Scheme code. A convenient way to call Scheme code from C is
using the function call_guile_fn_from_c which does all the
steps needed. Exceptions generated in Scheme code are translated to
trans-fails in C code, and vice versa. But this conversion is not
ideal, because VRR
trans-fails contain just strings whereas
GUILE exceptions contain arbitrary data; then an exception is
early converted to a string for trans-fail and the next exception in a row
is just a trans-fail exception.