; flip the top two items on the first stack.
; active stack after this: first
; this also preserves $input and $collect
;
⿲;   switch to second stack
⿳;   save $collect
⿶⿳; save $input
⿰;   switch back to first stack
⿱⿶; put top in $collect
⿲;   switch to 2nd stack
⿳;   push on 2nd stack
⿰;   switch to 1st stack
⿱;   put top(-1) in $input
⿶;   put top(-1) in $collect
⿲;   switch to 2nd stack
⿱;   put top to $input
⿳;   put (top-1) on 2nd stack
⿰;   switch to 1st stack
⿶⿳; put top from $input to $collect to 1st stack
⿲;   switch to 2nd stack
⿱⿶; put (top-1) in $collect
⿰⿳; switch to 1st stack and put $collect on 1st stack
⿲;   switch to second stack to restore $collect and $input
⿱⿶; pop old $input to $collect
⿰⿳; and push it on the first stack
⿲;   switch to second stack for $collect
⿱⿶; pop old $collect value to $collect
⿰;   switch back to first stack
⿱;   pop old $input value to $input