Add explanation section

EIPS/eip-1283.md

@@ -91,6 +91,59 @@ the following logic:
 Refund counter works as before -- it is limited to half of the gas
 consumed.
 
+## Explanation
+
+The new gas cost scheme for SSTORE is divided to three different
+types:
+
+* **No-op**: the virtual machine does not need to do anything. This is
+  the case if *current value* equals *new value*.
+* **Fresh**: this storage slot has not been changed, or has been reset
+  to its original value either on current frame, or on a sub-call
+  frame for the same contract. This is the case if *current value*
+  does not equal *new value*, and *original value* equals *current
+  value*.
+* **Dirty**: this storage slot has already been changed, either on
+  current frame or on a sub-call frame for the same contract. This is
+  the case if *current value* does not equal *new value*, and
+  *original value* does not equal *current value*.
+
+We can see that the above three types covers all possible variations
+of *original value*, *current value*, and *new value*.
+
+**No-op** is a trivial operation. Below we only consider cases for
+**Fresh** and **Dirty**.
+
+All initial (not-**No-op**) SSTORE on a particular storage slot starts
+with **Fresh**. After that, it will become **Dirty** if the value has
+been changed (either on current call frame or a sub-call frame for the
+same contract). When going from **Fresh** to **Dirty**, we charge the
+gas cost the same as current scheme.
+
+When entering a sub-call frame, a previously-marked **Dirty** storage
+slot will again becomes **Fresh**, but only for this sub-call
+frame. Note that we don't charge any more gases compared with current
+scheme in this case.
+
+In current call frame, a **Dirty** storage slot can be reset back to
+**Fresh** via a SSTORE opcode either on current call frame or a
+sub-call frame. For current call frame, this dirtiness is tracked, so
+we can issue refunds. For sub-call frame, it is not possible to track
+this dirtiness reset, so the refunds (for *current call frame*'s
+initial SSTORE from **Fresh** to **Dirty**) are not issued. In the
+case where refunds are not issued, the gas cost is the same as the
+current scheme.
+
+When a storage slot remains at **Dirty**, we charge 200 gas. In this
+case, we would also need to keep track of `R_SCLEAR` refunds -- if we
+already issued the refund but it no longer applies (*current value* is
+0), then removes this refund from the refund counter. If we didn't
+issue the refund but it applies now (*new value* is 0), then adds this
+refund to the refund counter. It is not possible where a refund is not
+issued but we remove the refund in the above case, because all storage
+slot starts with **Fresh** state, either on current call frame or a
+sub-call frame.
+
 ## Rationale
 
 This EIP mostly archives what EIP-1087 tries to do, but without the