'use strict';
                        
                        /**
                         * @class  EnvironmentVariables
                         * @module  Autodocs
                         */
                        
                        /**
                         * Test that an environment variable exists
                         *
                         * @method exists
                         * @for  EnvironmentVariables
                         * @param  {String} name The name of the environment variable
                         * @return {Boolean} `true` when environment variable is defined and **is not** an empty string
                         */
                        function existsEnvironmentVariable(name) {
                          return !!process.env[name];
                        }
                        
                        /**
                         * Enforce that an environment variable exists
                         *
                         * @method require
                         * @for  EnvironmentVariables
                         * @param  {String} name The name of the environment variable
                         * @throws  Throws and error when the environment variable does not exist
                         */
                        function requireEnvironmentVariable(name) {
                          if (!existsEnvironmentVariable(name)) {
                            throw new Error('Environment variable `'+name+'` not set');
                          }
                        }
                        
                        /**
                         * Set a default value for an environment variable if it does not exist
                         *
                         * @method default
                         * @for  EnvironmentVariables
                         * @param  {String} name The name of the environment variable
                         * @param  {String} value The default value
                         * @return {String} The value of the existing or newly set environment variable
                         */
                        function defaultEnvironmentVariable(name, value) {
                          if (!existsEnvironmentVariable(name)) {
                            process.env[name] = value;
                          }
                          return process.env[name];
                        }
                        
                        /**
                         * Substitute the value of other environment variables into the named one.
                         *
                         * For example:
                         *
                         * ```javascript
                         * process.env.MAJOR_VERSION = "0";
                         * process.env.MINOR_VERSION = "3";
                         * EnvironmentVariables.substitute('api/{{MAJOR_VERSION}}.{{MINOR_VERSION}}');
                         * ```
                         *
                         * ... will return `'api/0.3'`;
                         *
                         * Note that if this is done for multiple environment variables,
                         * the order in which they are done will affect the result.
                         * It is the responsibility of the caller to this function to ensure that
                         * the values to be substituted in are already fully resolved:
                         * this function **is not** recursive.
                         * Therefore, cycles are also not allowed.
                         * Any of these will result in indeterminate results, including errors.
                         *
                         * @method default
                         * @for  EnvironmentVariables
                         * @param  {String} name The name of the environment variable
                         * @return {String} The value of the existing or newly set environment variable
                         */
                        function substituteEnvironmentVariable(name) {
                          process.env[name] = process.env[name]
                            .replace(/{{[^{}]+}}/g, function(otherName) {
                              otherName = otherName.replace(/[{}]+/g, '');
                              requireEnvironmentVariable(otherName);
                              return process.env[otherName];
                            });
                          return process.env[name];
                        }
                        
                        module.exports = {
                          exists: existsEnvironmentVariable,
                          require: requireEnvironmentVariable,
                          'default': defaultEnvironmentVariable,
                          substitute: substituteEnvironmentVariable,
                        };