for _, b := range f.Blocks {
			var b0 *Block
			if debug > 1 {
				b0 = new(Block)
				*b0 = *b
				b0.Succs = append([]Edge{}, b.Succs...) // make a new copy, not aliasing
			}
			for i, c := range b.ControlValues() {
				for c.Op == OpCopy {
					c = c.Args[0]
					b.ReplaceControl(i, c)
				}
			}
			if rb(b) {
				change = true
				if debug > 1 {