Support Sketch-based Program Synthesis

[datvo06]

There are many cases where we would need to synthesize code to be placed in some specific context:

def train_a_nn(nn, param):
       <loss_function>

Here <loss_function> has to be placed exactly inside train_a_nn and have to be aware of the lexical context surrounding it.
I tried reproducing this behavior in different ways but each have a caveat:

1. Synthesizing the loss function and put in train_a_nn: then type checking would fail because the template lexical context is different.
2. Synthesizing the train_a_nn itself: risk of losing the part we don't want to lose.

[eb8680]

I'm not sure I understand the motivation here. Is there some design goal you have in mind that wouldn't be achieved much more straightforwardly by making loss_function an argument of train_a_nn?