logically evaluate a textual day of the week
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 Syntax

            GT_ISDOW( cDay ) -> lValue

 Arguments:

                <cDay>       is a character value for the weekday

 Returns:

                A logical value, .T. if the key is conditionally valid

 Description:

                GT_ISDOW() can be used to validate a character value as being a
                                   valid day of the week.

 Examples:


                ? "Day of Week ", cDay, " is ", GT_ISDOW( "Mon" )

                result : Day of Week Monday is   .T.


 Source: GT_ISDOW.PRG

 Author:  George Brennan