Function:   GRPTWRITE( <expN1> .. <expN3> ) --> NIL

Synopsis:   Writes a colored pixel at X,Y coordinate

            <expN1>  X coordinate
            <expN2>  Y coordinate

            <expN3>  color of pixel

            The function will write a "colored" pixel at a
            given X,Y coordinate.

Notes:      Make sure that the pixel is within the bounds of the
            valid Screenarea ( see b_maxx and b_maxy coordinates )
            which You set with UTINIT() to default values or using
            the GRINIT() function do specifiy Your own.

Examples:   GRPTWRITE( 100,200,3 )

Returns:    Nothing