Joseph Wright wrote:

 > I'd suggest the following:

> 

> New	Current Input		Output		Description

> 

> D	D	Varies		Varies		Do not use (kernel only)

> E	E	Single token	Unbraced	One expansion

> N	N	Single token	Unbraced	No expansion

> O	O	Single token	Braced		One expansion

> P	p	Parameters	n/a		Primitive TeX parameters

> W	w	Varies		Varies		"Weird" argument

> X	X	Single token	Braced		Full expansion

> 

> c	c	Braced tokens	Braced		Tokens to csname

> e	e	Braced tokens	Unbraced	Expand once

> f	F	Braced tokens	Braced		False branch

> n	n	Braced tokens	Braced		No expansion

> o	o	Braced tokens	Braced		One expansion

> s	f	Braced tokens	Braced		"Stop" expansion

> t	T	Braced tokens	Braced		True branch

> u	d	Braced tokens	Braced		Double expansion

> v	C	Braced tokens	Braced		To csname, expand once

> x	x	Braced tokens	Braced		Full expansion



Or perhaps by moving D:



New	Current Input		Output		Description



D	-	Single token	Braced		Double expansion

N	N	Single token	Unbraced	No expansion

O	O	Single token	Braced		One expansion

P	p	Parameters	n/a		Primitive TeX parameters

R (or K)D	Varies		Varies		Reserved/kernel only

U	E	Single token	Unbraced	Expand once unbraced

W	w	Varies		Varies		"Weird" argument

X	X	Single token	Braced		Full expansion



c	c	Braced tokens	Braced		Tokens to csname

u	e	Braced tokens	Unbraced	Expand once unbraced

f	F	Braced tokens	Braced		False branch

n	n	Braced tokens	Braced		No expansion

o	o	Braced tokens	Braced		One expansion

s	f	Braced tokens	Braced		"Special" expansion

t	T	Braced tokens	Braced		True branch

d	d	Braced tokens	Braced		Double expansion

e	C	Braced tokens	Braced		To csname, expand once

x	x	Braced tokens	Braced		Full expansion



I should point out that this leaves only u, U, N, R, W and P where the

output is not braced.  The later three I think there is no issue with.

I'd also suggest that "N" (and "n") mean "No change", so again I think

myself this is quite clear.  So the only "special" you really end up

with is u/U.  I'd suggest that one letter than behaves oddly can be

accepted. (It is a relatively rare one, at least for me at the moment.)



Also, to answer my own original question, this would then mean that

Morten's suggestion \exp_after:NN => \exp_args:NE would make sense. I

think that you simple have to accept that it is slightly odd (the whole

\exp_args:NE { \macro } business).

-- 

Joseph Wright