In Lil[0], this is how ordinary assignment syntax works. Implicitly defining a dictionary stored in a variable named "cat" with a field "age":

    cat.age:3
    # {"age":3}

    l:1,(list 2,list cat),4
    # (1,(2,{"age":3}),4)

    l[1][1].age:9
    # (1,(2,{"age":9}),4)
    cat
    # {"age":3}

    l:l,5
    # (1,(2,{"age":9}),4,5)