@Test
 public void setSymbolsAreReturned() {
   when(exec.getSymbol("x")).thenReturn("1");
   assertThat(variables.get("x"), is(equalTo("1")));
 }
 @Test
 public void unsetSymbolsReturnNull() {
   when(exec.getSymbol("n")).thenReturn(null);
   assertThat(variables.get("n"), is(equalTo(NULL)));
 }