begin_as_of

T1: begin → v1 read-write active={}
    set NextVersion = 2
    set TxnActive(1) = []

T1: set "other" = 0x01
    set TxnWrite(1, "other") = []
    set Version("other", 1) = 0x01

T2: begin → v2 read-write active={1}
    set NextVersion = 3
    set TxnActiveSnapshot(2) = {1}
    set TxnActive(2) = []

T2: set "key" = 0x02
    set TxnWrite(2, "key") = []
    set Version("key", 2) = 0x02

T2: commit
    del TxnWrite(2, "key")
    del TxnActive(2)

T3: begin → v3 read-write active={1}
    set NextVersion = 4
    set TxnActiveSnapshot(3) = {1}
    set TxnActive(3) = []

T3: set "key" = 0x03
    set TxnWrite(3, "key") = []
    set Version("key", 3) = 0x03

T4: begin as of 3 → v3 read-only active={1}

T4: scan ..
    "key" = 0x02

T4: set "foo" = 0x01 → Error::ReadOnly

T4: del "foo" → Error::ReadOnly

T1: commit
    del TxnWrite(1, "other")
    del TxnActive(1)

T3: commit
    del TxnWrite(3, "key")
    del TxnActive(3)

T4: scan ..
    "key" = 0x02

T5: begin as of 3 → v3 read-only active={1}

T5: scan ..
    "key" = 0x02

T4: rollback

T5: commit

T6: begin → v4 read-write active={}
    set NextVersion = 5
    set TxnActive(4) = []

T6: set "key" = 0x04
    set TxnWrite(4, "key") = []
    set Version("key", 4) = 0x04

T6: commit
    del TxnWrite(4, "key")
    del TxnActive(4)

T7: begin as of 4 → v4 read-only active={}

T7: scan ..
    "key" = 0x03
    "other" = 0x01

T8: begin as of 5 → Error::InvalidInput("version 5 does not exist")

T9: begin as of 9 → Error::InvalidInput("version 9 does not exist")

Engine state:
NextVersion = 5
TxnActiveSnapshot(2) = {1}
TxnActiveSnapshot(3) = {1}
Version("key", 2) = 0x02
Version("key", 3) = 0x03
Version("key", 4) = 0x04
Version("other", 1) = 0x01