---  { *Box {int !x; } int -> int #0}---------------------------
                 arg:rsi,box:rdi
0000 488bc6      mov   rax = rsi                      // def/empty1 #0
0003 4883f800    test  rax                           
0007 7d03        j>=   L3                             // L4
L4:
0009 48f7d8      neg   rax                           
L12:
L3:
                 :rax
000c 480107      add8  [rdi] += rax                  
000f c3          ret                                 
--- ---------------------------