Rightmost

Leftmost in reverse

Rightmost in reverse

Leftmost

$$P \leftrightarrow i,\,\,Q \leftrightarrow ii,\,\,R \leftrightarrow iv,\,\,S \leftrightarrow iii$$

$$P \leftrightarrow iii,\,\,Q \leftrightarrow i,\,\,R \leftrightarrow ii,\,\,S \leftrightarrow iv$$

$$P \leftrightarrow ii,\,\,Q \leftrightarrow iii,\,\,R \leftrightarrow i,\,\,S \leftrightarrow iv$$

$$P \leftrightarrow iv,\,\,Q \leftrightarrow i,\,\,R \leftrightarrow ii,\,\,S \leftrightarrow iii$$

Viable prefixes appear only at the bottom of the stack and not inside

Viable prefixes appear only at the top of the stack and not inside

The stack contains only a set of viable prefixes

The stack never contains viable prefixes

$$P - 2,\,\,Q - 3,\,\,R - 1,\,\,S - 4$$

$$P - 2,\,\,Q - 1,\,\,R - 4,\,\,S - 3$$

$$P - 2,\,\,Q - 4,\,\,R - 1,\,\,S - 3$$

$$P - 2,\,\,Q - 3,\,\,R - 4,\,\,S - 1$$