added immutable drops

contracts/token/ERC721/README.md

@@ -170,7 +170,7 @@ Mints `tokenId`, classifies it as `classId` and transfers to `recipient`.
 
  - **classId** - arbitrary class ID
  - **tokenId** - arbitrary token ID *(Should be unique)*
- - **recipient** - Wallet address to remove from the fee table
+ - **recipient** - Wallet address to send the token to
 
 ```js
 await nft.mint(100, 200 '0xabc123') // 1000 is 10.00%
@@ -201,6 +201,18 @@ await nft.unpause()
 Only the token owner can do the following actions. These are not included in the
 project's API and should be implemented on the client side.
 
+#### Burn
+
+Burns a token.
+
+`burn(uint256 tokenId)`
+
+ - **tokenId** - ex. 1
+
+```js
+await nft.burn(1)
+```
+
 ##### List
 
 Lists `tokenId` on the order book for `amount` in wei.
@@ -348,7 +360,7 @@ Returns the current supply size of `classId`
 `classSupply(uint256 classId)`
 
 ```js
-await nft.classFilled(100) //--> 1000
+await nft.classSupply(100) //--> 1000
 ```
 
 ##### Get the Fee of a Person

contracts/token/ERC721/abstractions/ERC721MultiClassDrop.sol

@@ -38,6 +38,10 @@ abstract contract ERC721MultiClassDrop is IERC721MultiClassDrop {
    * @dev Defines a redeemable class
    */
   function _drop(uint256 classId, bytes32 merkleroot) internal virtual {
+    require(
+      _root[classId] == 0,
+      "ERC721MultiClassDrop: Class has already been dropped"
+    );
     //save the merkle root
     _root[classId] = merkleroot;
   }