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#include <string.h> size_t strspn(const char *s1, const char *s2) { size_t r = 0; for (; *s1; s1++) { int e = 0; for (const char *t = s2; *t; t++) { if (*s1 == *t) { e = 1; break; } } if (!e) { break; } r++; } return r; }