# File lib/facet/lisp-format.rb, line 1524
        def execute(state)
          test = state.next_arg if colon_mod? or at_mod?
          if colon_mod?
            c = test ? 1 : 0
          elsif at_mod?
            if test
              state.push_back_arg
              c = 0
            end
          else
            n = param(0, state, nil)
            if n.nil?
              arg = state.next_arg
              if arg.respond_to? :to_int
                n = arg.to_int
              else
                arg_error 'argument is not an integral value'
              end
            end
            if n < @clauses.size
              c = n
            elsif @default != -1
              c = @default
            end
          end
          Format.execute_directives(state, @clauses[c]) unless c.nil?
        end