Contracts: include a string reference to the legal contract that the code is supposed to implement. Suggestion from Richard.

src/contracts/Cash.kt

@@ -27,6 +27,8 @@ class InsufficientBalanceException(val amountMissing: Amount) : Exception()
  * vaults can ignore the issuer/depositRefs and just examine the amount fields.
  */
 object Cash : Contract {
+    override val legalContractReference: String = "https://www.big-book-of-banking-law.gov/cash-claims.html"
+
     /** A state representing a cash claim against some institution */
     data class State(
             /** Where the underlying currency backing this ledger entry can be found (propagated) */

src/contracts/ComedyPaper.kt

@@ -23,6 +23,8 @@ val CP_PROGRAM_ID = SecureHash.sha256("comedy-paper")
 
 // TODO: Generalise the notion of an owned instrument into a superclass/supercontract. Consider composition vs inheritance.
 object ComedyPaper : Contract {
+    override val legalContractReference: String = "https://www.gotlines.com/jokes/1"
+
     data class State(
         val issuance: InstitutionReference,
         val owner: PublicKey,

src/core/Structures.kt

@@ -93,6 +93,9 @@ data class VerifiedSigned<out T : Command>(
 interface Contract {
     /** Must throw an exception if there's a problem that should prevent state transition. */
     fun verify(inStates: List<ContractState>, outStates: List<ContractState>, args: List<VerifiedSigned<Command>>, time: Instant)
+
+    /** Unparsed reference to the natural language contract that this code is supposed to express (usually a URL). */
+    val legalContractReference: String
 }
 
 /**