- * Generate a prime. We arrange to select a prime with the property
- * (prime % modulus) != residue (to speed up use in RSA).
+ * Generate a prime. We can deal with various extra properties of
+ * the prime:
+ *
+ * - to speed up use in RSA, we can arrange to select a prime with
+ * the property (prime % modulus) != residue.
+ *
+ * - for use in DSA, we can arrange to select a prime which is one
+ * more than a multiple of a dirty great bignum. In this case
+ * `bits' gives the size of the factor by which we _multiply_
+ * that bignum, rather than the size of the whole number.