ref: c3487a4b49b6988aad0e340ad9e986e4549b9317
dir: /sys/src/ape/lib/ap/gen/strspn.c/
#include <string.h>
#define N 256
size_t
strspn(const char *s, const char *b)
{
char map[N], *os;
memset(map, 0, N);
while(*b)
map[*(unsigned char *)b++] = 1;
os = s;
while(map[*(unsigned char *)s++])
;
return s - os - 1;
}