private IFormulaToken CompleteFunction(IEnumerable<IFormulaToken> tokens, int index)
{
var tokensAftwerwards = tokens.Skip(index);
tokensAftwerwards = CompleteBrackets(tokensAftwerwards);
tokensAftwerwards = CompleteFunctionsForward(tokensAftwerwards);
return tokensAftwerwards.FirstOrDefault();
}