# File lib/execjs/runtimes.rb, line 55
    def self.from_environment
      if name = ENV["EXECJS_RUNTIME"]
        if runtime = const_get(name)
          if runtime.available?
            runtime if runtime.available?
          else
            raise RuntimeUnavailable, "#{runtime.name} runtime is not available on this system"
          end
        elsif !name.empty?
          raise RuntimeUnavailable, "#{name} runtime is not defined"
        end
      end
    end