6.1.0130
+!
plus-store
CORE
 
( n | u a-addr -- )

Add n | u to the single-cell number at a-addr.

See:
3.3.3.1 Address alignment.

Testing:
T{  0 1ST !        ->   }T
T{  1 1ST +!       ->   }T
T{    1ST @        -> 1 }T
T{ -1 1ST +! 1ST @ -> 0 }T