Calculator()

         Displays a calculator

    Syntax:

         Calculator( [cColor], [cColorNum], [ColorMem] ) -> nResult

         cColor    : Box color
         cColorNum : Numbers color
         cColorMem : Memory numbers color

    Description:

         Display a calculator with selected colors.

    Return:

         The result of calculation if F10 is pressed or Nil if Esc is
         pressed.

    Example:

         nRes := Calculator ( "N/W", "G+/N", "R+/N" )