Backslash Calculus

Backslash Calculus is a programming language invented by User:Tailcalled after reading this reddit comment.

Syntax
Backslash Calculus is based on lambda calculus, but it only uses two characters: backslash and space. This is the basic syntax:

E = \ \ E   - abstraction | \ \\ E E - application | \\      - innermost variable | \\\     - the variable one level more out | ...     - ...

Computational Class
Backslash Calculus is clearly Turing-complete, even if one ignores its simple translation to lambda calculus. Here are the SKI combinators:

S = \ \ \ \ \ \ \ \\ \ \\ \\\\ \\ \ \\ \\\ \\ K = \ \ \ \ \\\ I = \ \ \\

Alternative alphabets
Tailcalled imagined a few alternative alphabets for Backslash Calculus.

Unreadable
One can replace backslash with ' and space with ". The SKI combinators will look like this with that alphabet:

S = '"'"'"'"'"'"'""'""'""'"""  K = '"'"'"'"'   I = '"'"''

Brainfuck
One can change the syntax a bit to make Backslash Calculus look more like brainfuck:

E0 = + E1 - | [ E0 E0 ] | .      | ,.       | ,,. <- unary-ish | ...   E1 = > E0 < | [ E1 E1 ] | .      | ,.       | ,,. <- unary-ish | ...

The SKI combinators will look like this:

S = +>+,,..][,..-<- K = +>[,.]<- I = +.-