updates to let-plus

I have just pushed some updates to the let-plus Github repository. The most important change is the handling of ignored values — let+ used to ignore nil, for example

(let+ (((a nil b) '(1 2 3)))   ; OLD SYNTAX, NO LONGER VALID
  ...)

would ignore the second element of the list. The implementation of let+ would simply replace nil with gensyms in the AST and declare them ignored. This caused some unexpected problems, for example in

(let+ (((a nil &optional (c nil c?)) '(1 2 3))) ; OLD SYNTAX, NO LONGER VALID
  ...)

nil is a value, not a placeholder for a variable. I could have extended let+ to work handle this, but that would have required destructuring the lambda lists internally instead of simply relying on destructuring-bind. Being lazy, I decided to change the syntax. From now on, users should use &ign to ignore variables. For example,

(let+ ((#(a &ign b) (vector 1 2 3)))
  (list a b)) ; => (1 3), no warning about the middle element not being used

The change is incompatible with the previous version (nil is no longer understood as being ignored and will most likely cause an error during macroexpansion), so you need to update your code if you are using let+. I am sorry about any inconvenience this may cause.

I also extended let+ to understand nesting in more places (when applicable, mostly for read-only forms). For example,

(let+ (((&slots-r/o ((&complex a b) bar))
        (make-instance 'foo :bar (complex 1 2))))
  (list 1 2)) ; => (1 2)

As usual, bug reports are welcome.