Quasi quoting

[Malcolm Wallace]

>  (ii) If "[pads|" is a lexeme, then some list comprehensions become  
> illegal,

I am not myself a TH or QQ user, but it has always bothered me  
slightly that the syntax for them steals some valid list comprehensions.

Of the alternative syntaxes you suggest...

> My gut feel is to go with [|pads| ... |].

... this one feels the nicest, because [|  |] is an ascii  
approximation of the common syntactic brackets used in semantic  
specifications.  In some ways, to make the correspondence even closer,

     pads [| ... |]

might be even better, although I realise that this might present new  
problems.

Regards,
     Malcolm