From: Brent Hilpert 
  This one:
 ...
 Reading more closely, the encoding has some relation back to BBC BASIC. 
Given this (from the documentation):
  Assembler directives
  --------------------
  #include <filename>     Includes the specified file.
  #ifndef <label>         Continue assembling if <label> is undefined.
  #ifdef <label>          Continue assembling if <label> is defined.
  #if <value>             Continue assembling if <value> is non-zero.
  #else                   Toggle assembly.
  #endif                  Continue assembling.
which is clearly C-related, I wonder if there is some relationship there (the
'%o' looks rather printf()-ish).
Although I note the documentation says "any valid value recognised by BBC
BASIC" - does BBC basic use the leading '%' notation for constants?
        Noel