bug in sb_strPat.ml

[arjunguha]

Subtraction is backward in one of these cases (reported by Ben Lerner):

let subtract v1 v2 = match v1, v2 with
| Set s1, Set s2 -> Set (S.subtract s1 s2)
| Reg r1, Reg r2 -> Reg (R.subtract r1 r2)
| Set s, Reg r
| Reg r, Set s -> Reg (R.subtract r (S.to_nfa s))

case three seems to be backwards...