in source/evp_override.c [962:972]
int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int len, const EVP_MD *md, ENGINE *impl) {
assert(hmac_ctx_is_valid(ctx));
if (md != NULL) {
if (key != NULL) {
ctx->md = md;
}
}
int rv;
__CPROVER_assume(rv == 1 || rv == 0);
return rv;
}