Letters that make postfix notation
s, a, o, f, n, p, t, i, x are letters that make 'postfix notation'
Words within postfix notation
tat, os, OS, postfix, ti, PO, Post, Tati, at, IX, TAT, on, ix, not, no, fix, post, notation, Io, ion, Po, At, No, Os, Ta, Ti are words within 'postfix notation'