Enhancement: Optional arguments of let-{rec,inline,block}

[nekketsuuu]

I noticed that optional arguments are sometimes not supported (syntax error).

Example:

% These are good.
let f ?:x y = ()
let-inline ctx \f ?:x y = read-inline ctx {}
let-block ctx +f ?:x y = read-block ctx '<>

% These are not.
let-rec f ?:x y = ()
let-inline \f ?:x y = {}
let-block +f ?:x y = '<>

So I asked this on Twitter in Japanese, and gfn-san answered that these three should be implemented. Therefore I post this issue for a record.

Environment

• Ubuntu 18.04
• OCaml 4.06.1
• SATySFi >=0.03 (current head 7ba1d05d258eac087e0439f3b731c96ca193c8a2)