Clarifications to EIP-663 (#6055)

* Rewrite the specification of EIP-663

* Some wording improvements

Co-authored-by: Paweł Bylica <chfast@gmail.com>

* Linter 😳

Co-authored-by: Paweł Bylica <chfast@gmail.com>

EIPS/eip-663.md

@@ -36,14 +36,20 @@ If the code is legacy bytecode, both of these instructions result in an *excepti
 
 If the code is valid EOF1, the following execution rules apply:
 
-These instructions are followed by an 8-bit immediate value, which we call `imm`, and can have a value of 0 to 255. We introduce the variable `n` which equals to `imm + 1`.
+ 1. These instructions are followed by an 8-bit immediate value, which we call `imm`, and can have a value of 0 to 255. We introduce the variable `n` which equals to `imm + 1`.
 
-If the current stack depth is less than `n`, then a stack underflow exception is issued. If the current stack depth is at the limit, a stack overflow exception is issued. In both of these cases the EVM stops and all gas is consumed.
+ 2. For `DUPN`:
 
-Otherwise
+   - If the current stack height is less than `n`, then a stack underflow exception is issued.
+   - If the current stack height is at the limit (1024), a stack overflow exception is issued.
+   - Otherwise the `n`'th stack item is duplicated at the top of the stack. (*Note: We use 1-based indexing here.*)
 
-- for `DUPN` the stack item at depth `n - 1` is duplicated at the top of the stack
-- for `SWAPN` the top stack item is swapped with the item at depth `n`
+ 3. For `SWAPN`:
+
+   - If the current stack height is less than `n + 1`, then a stack underflow exception is issued.
+   - Otherwise the `n + 1`th stack item is swapped with the top stack item.
+
+Clarification: the "stack underflow/overflow exception" means the EVM execution is halted and all gas is consumed.
 
 The gas cost for both instructions is set at 3.