In Python, it's possible to extend a list in a lazy way by using `itertools.chain`:

```
L = itertools.chain(L1, L2)

```

Is there a lazy map "gluing" operator? I.e.,

```
M = glue(M1, M2)

```

where

```
M['blah']

```

returns

```
M1['blah'] if 'blah' in M1 else M2['blah']

```

and, `M` has appropriate generators for `keys()` and `values()`.