Re: infix via code walker hook.

  1> (parse-infix '(sin a + b + c + d))
  (sin (+ (+ (+ a b) c)
          d))
>
But, if we put parentheses on the operand of sin, then it's no longer
the unary sin, but a function call with that argument:
>
  2> (parse-infix '(sin (a) + b + c + d))
  (+ (+ (+ (sin a) b)
c)
     d)

For someone coming from "statically typed functional programming", this
is *very* weird.
All those languages (Haskell/OCaml/SML/Coq/Idris/Agda/...) treat

    (sin a + b + c + d)

the same as

    (sin (a) + b + c + d)

I think it is already better than SRFI-105 curly braces and some
other systems.

A related effort was Honu:
https://users.cs.utah.edu/plt/publications/gpce12-rf.pdf


        Stefan