make unroller work for constants.

Sequential circuits with constant next states make
it quite awkward to assume no constants are passed
to unroll.At.  This commit checks for that.

Author: wsc1

logic/unroll.go

@@ -58,6 +58,14 @@ func (u *Unroll) At(m z.Lit, d int) z.Lit {
 		res = u.dmap[v][d]
 		goto Done
 	}
+	if v == 1 {
+		if m.IsPos() {
+			res = u.C.T
+		} else {
+			res = u.C.F
+		}
+		goto Done
+	}
 	n = u.S.nodes[v]
 	if n.b == z.LitNull {
 		// input