2SWAP 2ROT 2VARIABLE
2SWAP ( a,a b,b -- b,b a,a )  
ansi .6.1.0430 <CORE>
double-cell swap, see SWAP and 2DUP 
 simulate:
   : 2SWAP LOCALS| B1 B2 A1 A2 | B2 B1 A2 A1 ;

reference: p4_two_swap in ../src/core.c:0472, export CO 2SWAP