Other  operations treat variables numerically.  The `@' command permits numeric calculations to be performed and the result assigned to a vari-
       able.  Variable values are, however, always represented as (zero or more) strings.  For the purposes of numeric operations, the null string  is
       considered to be zero, and the second and subsequent words of multi-word values are ignored.

       
% set a=0
% a += 1
a: Command not found.

% @ a += 1
% echo $a
1

% @ a += 1
% echo $a
2