def opt(name, desc = "", opts = {}, &b)
opts[:callback] ||= b if block_given?
opts[:desc] ||= desc
o = Option.create(name, desc, opts)
raise ArgumentError, "you already have an argument named '#{name}'" if @specs.member? o.name
raise ArgumentError, "long option name #{o.long.inspect} is already taken; please specify a (different) :long" if @long[o.long]
raise ArgumentError, "short option name #{o.short.inspect} is already taken; please specify a (different) :short" if @short[o.short]
@long[o.long] = o.name
@short[o.short] = o.name if o.short?
@specs[o.name] = o
@order << [:opt, o.name]
end