public string NormalizeExpression(string expression)
{
// the non dynamic way.
var pivotExpression = PivotsExpression.ReadExpression(_pivots, (string item, out string choice, out string name) => {
Pivot pivot;
var result = GetChoice(item, out choice, out pivot);
name = pivot.Name;
return(result);
}, expression);
return(_normalizedPivots.GetOrAdd(pivotExpression, () => expression));
}