Support of Between function in flee


I am creating a C# application and using.
I know flee can be used for built-in system functions.
However, I want to use the Between function with flee. "The BETWEEN operator is used to select values within a range"as below:

public static class Ext
    public static bool Between(this int source, int low, int high)
        return source.CompareTo(low) >= 0 && source.CompareTo(high) <= 0;
Is there any way I can work with my own custom functions?