code: func ComposeEnviron(env types.Environment) []string {
  	var composed []string
  
  	for dk, dv := range defaultEnv {
  		if _, exists := env.Get(dk); !exists {
  			composed = append(composed, fmt.Sprintf("%s=%s", dk, dv))
  		}
  	}
  
  	for _, e := range env {
  		composed = append(composed, fmt.Sprintf("%s=%s", e.Name, e.Value))
  	}
  	return composed
  }
docs: // ComposeEnviron formats the environment into a slice of strings,
  // each of the form "key=value".  The minimum required environment
  // variables by the appc spec will be set to sensible defaults here if
  // they're not provided by env.