a86: add syntax on modes and nests to ga68.vw Signed-off-by: Jose E. Marchesi <jemarch@gnu.org> gcc/algol68/ChangeLog * ga68.vw (Modes and nests): Incorporate syntax from the RR chapter 7. (cherry picked from commit e123cd977b90082c71ae98cb816f607e5b9a8fd7)