internal virtual bool ExistsInFirst(TFirst value)
 {
     if (FirstToSecondDictionary.ContainsKey(value))
     {
         return(true);
     }
     return(false);
 }
        // *******************************************************************

        /// <summary>
        /// This method indicates wether a value exists as a 1st relation.
        /// </summary>
        /// <param name="value">The value to use for the operation.</param>
        /// <returns>true if the value exists; false otherwise.</returns>
        public virtual bool ExistsInFirst(
            TFirst value
            )
        {
            // Validate the parameter before attempting to use it.
            new Guard().ThrowIfNull(value, nameof(value));

            // Does the value exist in the internal dictionary?
            if (FirstToSecondDictionary.ContainsKey(value))
            {
                return(true);
            }

            // Not there.
            return(false);
        }