at(index, values): this 
    // inserts values at index.  index can be negative or positive.
    // If positive, existing items beginning at that index will be pushed to 
    // the right to make room.  If negative, existing items ending at that 
    // index will be pushed to the left to make room.
    
middle(values, offset? = 0): this
    // inserts values in middle of this.data .
    // By default, if the array has odd number of items, values will be 
    // inserted just before the middle item. If you want to change the insert 
    // position, set the optional offset parameter to + or - whatever integer 
    // you want.

The methods below are not important to know about in order to use this
class. They're inherited from BaseClass .

protected   _createGetterAndOrSetterForEach(
        propertyNames: string[],
        configuration: IGetterSetterConfiguration
       ) : void
    /*********************
    Use this method when you have a bunch of properties that need getter and/or 
    setter functions that all do the same thing. You pass in an array of string 
    names of those properties, and the method attaches the same getter and/or 
    setter function to each property.
    IGetterSetterConfiguration is this object:
    {
        get_setterFunction?: (
             propertyName: string, index?: number, propertyNames?: string[]
        ) => Function,
        // get_setterFunction takes the property name as first argument and 
        // returns the setter function.  The setter function must take one 
        // parameter and return void.
        
        get_getterFunction?: (
             propertyName: string, index?: number, propertyNames?: string[]
        ) => Function
        // get_getterFunction takes the property name as first argument and 
        // returns the getter function.  The getter function must return something.
    }
    *********************/ 
       
       
protected   _returnThis_after(voidExpression: any) : this
    // voidExpression is executed, then function returns this.
    // Even if voidExpression returns something, the returned data isn't used.

protected   _errorIfPropertyHasNoValue(
                property: string, // can contain dot-notation, i.e., 'property.subproperty'
                propertyNameInError? = ''
            ) : void
    // If value of this[property] is undefined or null, it triggers fatal error:
    // `The property "${propertyNameInError}" has no value.`