+ void (*letter)(struct termp *, int);
+ void (*begin)(struct termp *);
+ void (*end)(struct termp *);
+ void (*endline)(struct termp *);
+ void (*advance)(struct termp *, size_t);
+ size_t (*width)(const struct termp *, int);
+ double (*hspan)(const struct termp *,
+ const struct roffsu *);