func (b *Block) AddControl(v *Value) {
	i := b.NumControls()
	b.Controls[i] = v // panics if array is full
	v.Uses++
}

// ReplaceControl exchanges the existing control value at the index provided
// for the new value. The index must refer to a valid control value.
func (b *Block) ReplaceControl(i int, v *Value) {
	b.Controls[i].Uses--
	b.Controls[i] = v
	v.Uses++
}

	for _, v := range b.Values {
		for i, a := range v.Args {
			if a == old {
				v.SetArg(i, new)
			}
		}
	}
	for i, v := range b.ControlValues() {
		if v == old {
			b.ReplaceControl(i, new)
		}
	}
}

// moveTo moves v to dst, adjusting the appropriate Block.Values slices.
// The caller is responsible for ensuring that this is safe.
// i is the index of v in v.Block.Values.

				if a.Op == OpSPanchored || opcodeTable[a.Op].nilCheck {
					v.SetArg(i, a.Args[0])
				}
			}
		}
		for i, c := range b.ControlValues() {
			if c.Op == OpSPanchored || opcodeTable[c.Op].nilCheck {
				b.ReplaceControl(i, c.Args[0])
			}
		}
	}
	for _, b := range f.Blocks {
		i := 0
		for _, v := range b.Values {
			if v.Op == OpSPanchored {
				// Free this value
				if v.Uses != 0 {