foo=[]; bar=['a', 'b']
 key   0:3   ^-^                     foo
 grp   5:5       ^                   
 val   5:5       ^                   
pair   0:5   ^---^                   foo=[
 key   8:11          ^-^             bar
quot  14:15                ^         a
 val  14:15                ^         a
quot  19:20                     ^    b
 val  19:20                     ^    b
 grp  14:20                ^----^    a', 'b
 val  14:20                ^----^    a', 'b
pair   8:20          ^----------^    bar=['a', 'b
msg         :foo=[]; bar=['a', 'b']
format      :foo=[#]; bar=[#]
{
    "foo": null,
    "bar": [
        "a",
        "b"
    ]
}