@Test(expected = ChildDoesNotExistException.class)
public void replaceChildBad1() throws Exception {

  VariableIdentifierExpr v = new VariableIdentifierExpr("v");
  VariableIdentifierExpr w = new VariableIdentifierExpr("v");
  Expr pe = new ParenExpr(v);
  pe.replaceChild(w, v);
}