((dynstack)->base + SCM_DYNSTACK_HEADER_LEN)

#define SCM_DYNSTACK_CAPACITY(dynstack) \
  ((dynstack)->limit - (dynstack)->base)
#define SCM_DYNSTACK_SPACE(dynstack) \
  ((dynstack)->limit - (dynstack)->top)
#define SCM_DYNSTACK_HEIGHT(dynstack) \
  ((dynstack)->top - (dynstack)->base)

#define SCM_DYNSTACK_HAS_SPACE(dynstack, n) \
  (SCM_DYNSTACK_SPACE (dynstack) >= n + SCM_DYNSTACK_HEADER_LEN)

typedef enum {
  SCM_F_DYNSTACK_FRAME_REWINDABLE = (1 << SCM_DYNSTACK_TAG_FLAGS_SHIFT)
} scm_t_dynstack_frame_flags;

typedef enum {
  SCM_F_DYNSTACK_WINDER_EXPLICIT = (1 << SCM_DYNSTACK_TAG_FLAGS_SHIFT)
} scm_t_dynstack_winder_flags;

typedef enum {
  SCM_F_DYNSTACK_PROMPT_ESCAPE_ONLY = (1 << SCM_DYNSTACK_TAG_FLAGS_SHIFT)
} scm_t_dynstack_prompt_flags;

typedef void (*scm_t_guard) (void *);