? #define A(X, Y) Y X
? #define B 1 ## 2 ## 3
? .singlestep  A(B, 2)
current:  [1 ## 2 ## 3] in A( _ , 2 )
? step
current:  [12 ## 3] in A( _ , 2 )
? step
current:  [123] in A( _ , 2 )
? step
current:  [2] in A( 123, _ )
? step
expanding A with ( 123, 2 )
current:  [2] in _ 123
?