How can you read and understand the source of *naryFun in functional.d?

[Tom]

Hi,
I am learning D for some time. I come from background of C, C# and Python.
When I saw the ways to use std.algorithem's functions, I have noticed that the
input lambda's can be writen as strings. Somewhat like the pythonic "exec". I
went to the source of this feature in functional.d
("https://github.com/D-Programming-Language/phobos/blob/master/std/functional.d").
The functions unaryFun and binaryFun. Is there a way I can read them and
understand them easily? or maybe I missed something?