.Nat: ClassType
.Nat.
    '''
    Execute `proc!` `self` times.
    '''
    '''erg
    10.times! do!:
        print! "hello"

    print_hello!() = print! "hello"
    10.times! print_hello!
    '''
    times!: (self: .Nat, proc!: () => NoneType) => NoneType