Freeeze for Marvin3

Today, I added a keyword called freeze to Marvin3. Basically, when called inside a routine, it pops the top element from the stack and replaces routine's code with a word that pushes that value onto the stack. This way we can create a write-once routine, which is similar to C#'s readonly modifier (value is allowed to be set once). Consider the following example:

:YOUR-NUMBER input int freeze ;

"Enter a number:" prints YOUR-NUMBER
YOUR-NUMBER " is a nice number!" cat println

Notice that, although YOUR-NUMBER is a routine, when called, it just pushes the same value determined when it is first called.

Here is a more complex one. The following set of class and routines implement a generator that returns subsequent numbers starting from 1 (then 2, 3, ...):

@__generate 0 !n [%n incr ^n] dup ;
:_generate __generate freeze ;
:generate _generate call ;

The Python equivalent is (although using generators would be better):

class _generate:
    def __init__(self):
        self.n = 0
    def __call__(self):
        self.n += 1
        return self.n

generate = _generate()