Merge #602: Document And/Or vectors contain 2 elements

47c8776 Document And/Or vectors contain 2 elements (Tobin C. Harding)

Pull request description:

  It is not immediately obvious that the `Concrete::And` and `Or` vectors contain exactly 2 elements, add code comment to document it.

ACKs for top commit:
  apoelstra:
    ACK 47c8776

Tree-SHA512: 477c1a3b36a4c5632f35433cbcb4bcafb4d005acdff6a82785ee7c175aa243ce87439c8c6d71859face17035fc75ba232d2b42854edc7194b9b99134168dac34

src/policy/concrete.rs

@@ -38,6 +38,9 @@ const MAX_COMPILATION_LEAVES: usize = 1024;
 /// Concrete policy which corresponds directly to a miniscript structure,
 /// and whose disjunctions are annotated with satisfaction probabilities
 /// to assist the compiler.
+// Currently the vectors in And/Or are limited to two elements, this is a general miniscript thing
+// not specific to rust-miniscript. Eventually we would like to extend these to be n-ary, but first
+// we need to decide on a game plan for how to efficiently compile n-ary disjunctions
 #[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub enum Policy<Pk: MiniscriptKey> {
     /// Unsatisfiable.