Lars Hellström wrote:
> Joseph Wright skrev:
>> I wonder if anyone can make a suggestion on how to solve a problem I'm
>> finding rather tricky with my "keys" efforts.  The issue is adding code
>> to an existing macro.  There are a number of scenarios, but I control
>> the initial definition, 
> In that case, I would *strongly* suggest designing your macro to be
> extendable to begin with -- e.g. equip it with a helper macro that acts
> as hook. 

This was my first idea.  However, I'm struggling with the
implementation, hence the question.

> There might in rare cases be a need to pass parameters to this
> hook (although storing them in helper macros is typically easier), but
> there should never be a need to use delimited arguments.

The idea here is that the user provides three things when creating a key:
1) Key name
2) Argument spec.
3) Code

Obviously, the key name is used in a csname to make things work, while
initially the arg spec and code are used to create the macro.  The point
is that when adding code, anything from the arg spec. (which could
contain any number of parameters) is fair game.  So my attempts to
create a simple hook:

\def:cpx {<key>} <arg spec.> {\exp_not:N\prehook <code> \exp_not:posthook}

fail when the added code contains #1, #2, ... My current approach at
least does not give a TeX error under these circumstances (I've added a
custom one to warn of a problem).

I'm sure I'm mossing something, but my knowledge of TeX's approach to
doubling # characters, etc., is clearly insufficient to find it.

Joseph Wright