Question: smtprocessor new1 state & fnc[1]

[wadro]

This is state change diagram in source file (smtprocessor.circom):

Insert to a used leaf.
=====================

  STATE                 OLD STATE                                       NEW STATE
  =====                 =========                                       =========


                         oldRoot                                          newRoot
                            ▲                                               ▲
                            │                                               │
          ┌───────┐     ┏━━━┻━━━┓                         ┌───────┐     ┏━━━┻━━━┓
   top    │Sibling├────▶┃ Hash  ┃◀─┐                    │Sibling├────▶┃ Hash  ┃◀─┐
          └───────┘     ┗━━━━━━━┛  │                      └───────┘     ┗━━━━━━━┛  │
                                   │                                               │
                                   │                                               │
                               ┏━━━┻━━━┓   ┌───────┐                           ┏━━━┻━━━┓   ┌───────┐
   top                  ┌─────▶┃ Hash  ┃◀──┤Sibling│                  ┌─────▶┃ Hash  ┃◀──┤Sibling│
                        │      ┗━━━━━━━┛   └───────┘                    │      ┗━━━━━━━┛   └───────┘
                        │                                               │
                        │                                               │
        ┌───────┐   ┏━━━┻━━━┓                           ┌───────┐   ┏━━━┻━━━┓
   top  │Sibling├──▶┃ Hash  ┃◀─────┐                  │Sibling├──▶┃ Hash  ┃◀─────┐
        └───────┘   ┗━━━━━━━┛      │                    └───────┘   ┗━━━━━━━┛      │
                                   │                                               │
                                   │                                               │
                              ┌────┴────┐                                      ┏━━━┻━━━┓   ┌───────┐
   bot                        │Old1Leaf │                               ┌─────▶┃ Hash  ┃◀──┼─  0   │
                              └─────────┘                               │      ┗━━━━━━━┛   └───────┘
                                                                        │
                                                                        │
                     ┏━━━━━━━┓                          ┌───────┐   ┏━━━┻━━━┓
   bot               ┃ Hash  ┃                          │   0  ─┼──▶┃ Hash  ┃◀─────┐
                     ┗━━━━━━━┛                          └───────┘   ┗━━━━━━━┛      │
                                                                                   │
                                                                                   │
                     ┏━━━━━━━┓                                                 ┏━━━┻━━━┓   ┌───────┐
   bot               ┃ Hash  ┃                                          ┌─────▶┃ Hash  ┃◀──│   0   │
                     ┗━━━━━━━┛                                          │      ┗━━━━━━━┛   └───────┘
                                                                        │
                                                                        │
                     ┏━━━━━━━┓                        ┌─────────┐   ┏━━━┻━━━┓   ┌─────────┐
  new1               ┃ Hash  ┃                        │Old1Leaf ├──▶┃ Hash  ┃◀──│New1Leaf │
                     ┗━━━━━━━┛                        └─────────┘   ┗━━━━━━━┛   └─────────┘


                     ┏━━━━━━━┓                                      ┏━━━━━━━┓
   na                ┃ Hash  ┃                                      ┃ Hash  ┃
                     ┗━━━━━━━┛                                      ┗━━━━━━━┛


                     ┏━━━━━━━┓                                      ┏━━━━━━━┓
   na                ┃ Hash  ┃                                      ┃ Hash  ┃
                     ┗━━━━━━━┛                                      ┗━━━━━━━┛

In bot state, Old1Leaf suddenly is suddenly changed to Hash of 0, and in new1 state, old1leaf value and new1leaf value are combined to hash.
In my understanding, when old1leaf is changed to new1leaf, new SMT root hash have to combined only with new1leaf and new child, comparing with previous old SMT root hash is combined only with old1leaf and old child.
Is this kind of insertion case? But why were nodes suddenly created and inserted in the middle of highest bot level and new1 level?
I need a little more explanation. I'm trying to utilize this in my rollup code, but I've been looking at the SMTprocessor code implementation for 3 days trying to understand the internals.

And refering to this code block:

component topSwitcher = Switcher();

    topSwitcher.sel <== fnc[0]*fnc[1];
    topSwitcher.L <== levels[0].oldRoot;
    topSwitcher.R <== levels[0].newRoot;

    component checkOldInput = ForceEqualIfEnabled();
    checkOldInput.enabled <== enabled;
    checkOldInput.in[0] <== oldRoot;
    checkOldInput.in[1] <== topSwitcher.outL;

Contrary to its initial intention, I think it does not seem for fnc[1] to be used very meaningfully.
Because I cannot find any DELETE mechanism described below block:

Fnction
fnc[0]  fnc[1]
0       0             NOP
0       1             UPDATE
1       0             INSERT
1       1             DELETE

in this block:

    component keysOk = MultiAND(3);
    keysOk.in[0] <== 1-fnc[0];
    keysOk.in[1] <== fnc[1];
    keysOk.in[2] <== 1-areKeyEquals.out;

    keysOk.out === 0;

When fnc[0] == 0, fnc[1] == 1, areKeyEquals.out == 0, it will cause circuit error.
In other words, it appears to be code to check that the keys are not different from each other.
If the keys are the same, this constraint must pass. Even if the keys are different, (0,0) NOP, (1,0) INSERT, and (1,1) DELETE will pass.
Why did you create this constraint with MultiAND?
I looked and studied as much as I could, but I couldn't understand it in the end because I lacked understanding.
This is an amazing library, but it's unfortunate that the users like me are inexperienced... :(
Thank you for your work.