I suspect this is because

    def f(x, y): x+y

is valid one-line syntax for a regular function definition (that in this case computes `x+y` and returns `None` becaus Python does not have implicit returns). Maybe the parser could distinguish between them by saying that "if the function definition is all on one line and the function is missing its name, it's a lambda":

    def f(x,y): return x+y
    g = def x,y: x+y
    assert f(3,4) == g(3,4)
    assert f != g

 That said I have no 

Nothing gives me the willies in mathematical code like writing out Greek letters. I don't know why, it just does. The fact that the otherwise excellent Stan (https://mc-stan.org) only accepts ASCII variable names is depressing. I would love to be able to actually write `map(λ x: x+7, range(5))`. I think a big reason why people don't use `map`/`reduce`/etc. more is that writing anonymous functions is still verbose (`lambda` is just too long a word) and relatively hard to read compared to list comprehension. Better still a syntax like `x -> x+7` would be ideal, since I'm pretty sure `->` would be a unique syntax element.

It's because lambda technically isn't an anonymous function. It's more like an anonymous expression. It probably helps to avoid confusion for people trying to do things that can't be expressions in it. E.g.:

    def (x,y): x += y