Function:    MouseChoice()

               The MouseChoice() function is a function similar to
               Achoice(), supporting the Mouse and handling some things
               other ways, more compatible with the other 'high level'
               Mouse routines.

               Next to normal editing keys (see "MouseTrap"), MouseChoice()
               also reacts to normal keys (alphanumeric, letters etc..), by
               positionning the bar on the next item whose first letter
               matches the key pressed (or returned by the Trap function).

  Syntax:      MouseChoice(nTop,nLeft,nBottom,nRight,aVal[,bTrap])

  Arguments:   The four first arguments define the table's position
               on the screen. aVal is an array of displayable items,
               which is displayed within the screen box. bTrap is an
               optionnal trap function, see MouseTrap.

  Returns:     This function returns the following: 0 when the user
               pressed Esc (or pressed the right button) and current
               selection number when selected with Enter (or Left
               button).

  Usage:       See the DEMO programs that have been supplied with
               the Expand library for some detailed examples on how to
               use this function.