code: @Override
      public String lookup(String key) {
          final String value = System.getenv(key);
  
          if (value == null && strict) {
              throw new UndefinedEnvironmentVariableException("The environment variable '" + key
                      + "' is not defined; could not substitute the expression '${"
                      + key + "}'.");
          }
  
          return value;
      }
docs: {@inheritDoc}
  
  @throws UndefinedEnvironmentVariableException if the environment variable doesn't exist and strict behavior
  is enabled.