code: func LookupEnv(ctx context.Context, key string) (string, bool) {
  	if s, ok := ctx.Value(ctxOSEnviron).([]string); ok {
  		for _, v := range s {
  			p := strings.SplitN(v, "=", 2)
  			if len(p) > 0 && strings.EqualFold(p[0], key) {
  				if len(p) > 1 {
  					return p[1], true
  				}
  				return "", true
  			}
  		}
  	}
  	if f, ok := ctx.Value(ctxOSLookupEnvKey).(lookupEnvFunc); ok {
  		if v, ok := f(key); ok {
  			return v, true
  		}
  	}
  	return os.LookupEnv(key)
  }
docs: // LookupEnv returns the value of the provided environment variable by:
  //
  //   1. Inspecting the context for a key "os.Environ" with a string
  //      slice value. If such a key and value exist then the string slice
  //      is searched for the specified key and if found its value is returned.
  //
  //   2. Inspecting the context for a key "os.LookupEnv" with a value of
  //      func(string) (string, bool). If such a key and value exist then the
  //      function is used to attempt to discover the key's value. If the
  //      key and value are found they are returned.
  //
  //   3. Returning the result of os.LookupEnv.