Final draft for IERC777.

contracts/token/ERC777/IERC777.sol

@@ -2,6 +2,12 @@ pragma solidity ^0.5.0;
 
 /**
  * @dev Interface of the ERC777Token standard as defined in the EIP.
+ *
+ * This contract uses the
+ * [`ERC1820Registry`](https://eips.ethereum.org/EIPS/eip-1820) to let token
+ * holders and recipients react to token movements by using setting implementers
+ * for the associated interfaces in said registry. See `IERC1820Registry` and
+ * `ERC1820Implementer`.
  */
 interface IERC777 {
     /**
@@ -43,6 +49,12 @@ interface IERC777 {
      * `operatorData`. See `IERC777Sender` and `IERC777Recipient`.
      *
      * Emits a `Sent` event.
+     *
+     * Requirements
+     *
+     * - the caller must have at least `amount` tokens.
+     * - `to` cannot be the zero address.
+     * - if `to` is a contract, it must implement the tokensReceived interface.
      */
     function send(address to, uint256 amount, bytes calldata data) external;
 
@@ -54,6 +66,10 @@ interface IERC777 {
      * will be called with `data` and empty `operatorData`. See `IERC777Sender`.
      *
      * Emits a `Burned` event.
+     *
+     * Requirements
+     *
+     * - the caller must have at least `amount` tokens.
      */
     function burn(uint256 amount, bytes calldata data) external;
 
@@ -111,6 +127,14 @@ interface IERC777 {
      * `operatorData`. See `IERC777Sender` and `IERC777Recipient`.
      *
      * Emits a `Sent` event.
+     *
+     * Requirements
+     *
+     * - `from` cannot be the zero address.
+     * - `from` must have at least `amount` tokens.
+     * - the caller must be an operator for `from`.
+     * - `to` cannot be the zero address.
+     * - if `to` is a contract, it must implement the tokensReceived interface.
      */
     function operatorSend(
         address from,
@@ -128,6 +152,12 @@ interface IERC777 {
      * will be called with `data` and `operatorData`. See `IERC777Sender`.
      *
      * Emits a `Burned` event.
+     *
+     * Requirements
+     *
+     * - `from` cannot be the zero address.
+     * - `from` must have at least `amount` tokens.
+     * - the caller must be an operator for `from`.
      */
     function operatorBurn(
         address from,