Expr eX = evalArg(args[2]);
if (!(eX instanceof ExprArray))
return ExprError.VALUE;
double forecastX = ((ExprNumber) eF).doubleValue();
ExprArray knownY = (ExprArray) eY;
ExprArray knownX = (ExprArray) eX;
if (knownY.length() != knownX.length())
return ExprError.NA;
Expr aeY = AVERAGE.average(knownY);
if (aeY instanceof ExprError)
return aeY;