-- implement \s (physical font size)
- \sN - set to N
- \s+N - increment by N, 1 <= N <= 8
- \s-N - decrement by N, 1 <= N <= 8
- \s0 - restore previous size
- \s(nn \s+-(nn \s'+-nn' \s+-'nn' \s[+-nn] \s+-[nn] - disambiguate
- registers: .s .ps .sr .psr
- used by e.g. cu(1), tip(1)
-