PopUp([<nTop>], [<nLeft>], [<nBottom>], [<nRight>])
        --> oPopUp

 Arguments

     <nTop> is a numeric value that indicates the top screen row of the
     pop-up menu.  If omitted, PopUpMenu:top is set to an appropriate value
     relative to <nBottom> that allows as many items as possible to show.  If
     <nBottom> is also omitted, PopUpMenu:top is set to center the menu
     vertically on the screen.  The default value is determined the first
     time the pop-up menu is displayed.

     When the pop-up menu is a child of another menu, its top variable will
     be automatically set by the parent menu regardless of whether <nTop> is
     omitted.

     <nLeft> is a numeric value that indicates the left screen column of
     the pop-up menu.  If omitted, PopUpMenu:left is set to an appropriate
     value relative to <nRight> that allows as many menu columns as possible
     to show.  If <nRight> is also omitted, PopUpMenu:left is set to center
     the menu horizontally on the screen.  The default value is determined
     the first time the pop-up menu is displayed.

     When the pop-up menu is a child of another menu, its left variable will
     be automatically set by the parent menu regardless of whether <nLeft> is
     omitted.

     <nBottom> is a numeric value that indicates the bottom screen row of
     the pop-up menu.  If omitted, PopUpMenu:bottom is set to an appropriate
     value relative to <nTop> that allows as many items as possible to show.
     If <nTop> is also omitted, PopUpMenu:bottom is set to center the menu
     vertically on the screen.  The default value is determined the first
     time the pop-up menu is displayed.

     When the pop-up menu is a child of another menu, its bottom variable
     will be automatically set by the parent menu regardless of whether
     <nBottom> is omitted.

     <nRight> is a numeric value that indicates the right screen column
     of the pop-up menu.  If omitted, PopUpMenu:right is set to an
     appropriate value relative to <nLeft> that allows as many menu columns
     as possible to show.  If <nLeft> is also omitted, PopUpMenu:right is set
     to center the menu horizontally on the screen.  The default value is
     determined the first time the pop-up menu is displayed.

     When the pop-up menu is a child of another menu, its right variable will
     be automatically set by the parent menu regardless of whether <nRight>
     is omitted.

 Returns

     Returns a PopUpMenu object when all of the required arguments are
     present; otherwise, PopUp() returns NIL.