clarify.
diff --git a/doc/crypto/DSA_set_method.pod b/doc/crypto/DSA_set_method.pod
index 0b13ec9..edec464 100644
--- a/doc/crypto/DSA_set_method.pod
+++ b/doc/crypto/DSA_set_method.pod
@@ -62,12 +62,13 @@
int (*dsa_do_verify)(const unsigned char *dgst, int dgst_len,
DSA_SIG *sig, DSA *dsa);
- /* compute rr = a1^p1 * a2^p2 mod m. May be NULL */
+ /* compute rr = a1^p1 * a2^p2 mod m (May be NULL for some
+ implementations) */
int (*dsa_mod_exp)(DSA *dsa, BIGNUM *rr, BIGNUM *a1, BIGNUM *p1,
BIGNUM *a2, BIGNUM *p2, BIGNUM *m,
BN_CTX *ctx, BN_MONT_CTX *in_mont);
- /* compute r = a ^ p mod m. May be NULL */
+ /* compute r = a ^ p mod m (May be NULL for some implementations) */
int (*bn_mod_exp)(DSA *dsa, BIGNUM *r, BIGNUM *a,
const BIGNUM *p, const BIGNUM *m,
BN_CTX *ctx, BN_MONT_CTX *m_ctx);
