> 	I thought that, historically, Unix has used six character identifiers
> 	(later seven chars) and no underscores for lack of space.
>My understanding was that the first six were required to be UNIQUE.  That
>is you could have as many characters as you cared to use, but the compiler
>was only required to use the first 6.  

Does anyone have an original-edition C Programming Language handy?

As I recall, the V6/V7 PDP-11 linker truncated external symbols at 7
characters, preserving case; but the linker used on one of the other
machines V6 ran on insisted on 6 characters monocase.  I'm almost certain
this is discussed in the first or second chapter of the original CPL, but
my carefully-archived(*) copy is at home.

(*) and I don't know in which pile it is carefully archived.