Gregg Irwin explaining How set works in Red

gistfile1.txt

 o: object [a: 1 b: 2 c: 3]
== make object! [
    a: 1
    b: 2
    c: 3
]
>> set o object [a: 11 b: 12]
== make object! [
    a: 11
    b: 12
]
>> o
== make object! [
    a: 11
    b: 12
    c: 3
]
>> set/only o object [a: 11 b: 12]
== make object! [
    a: 11
    b: 12
]
>> o
== make object! [
    a: make object! [
        a: 11
        b: 12
    ]
    b: make object! [
        a: 11
        b: 1...
>> probe o
make object! [
    a: make object! [
        a: 11
        b: 12
    ]
    b: make object! [
        a: 11
        b: 12
    ]
    c: make object! [
        a: 11
        b: 12
    ]
]