func (po *poset) upushneq(idx1 uint32, idx2 uint32) {
	po.undo = append(po.undo, posetUndo{typ: undoNonEqual, ID: ID(idx1), idx: idx2})
}

// upushalias pushes a new undo pass for aliasing two nodes
func (po *poset) upushalias(id ID, i2 uint32) {
	po.undo = append(po.undo, posetUndo{typ: undoAliasNode, ID: id, idx: i2})
}

// upushconst pushes a new undo pass for a new constant