@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)));
}