@Override
public boolean equals(Object other) {
return other instanceof Evaluation
&& (expression != null && expression.equals(((Evaluation) other).expression))
&& (result != null && result.equals(((Evaluation) other).result));
}
@Override
public int hashCode() {
return (expression != null ? expression.hashCode() : 0)
+ (result != null ? result.hashCode() : 0);
}