What does the snake mean in Susan's garden in "To Room Nineteen"?

In "To Room Nineteen," Doris Lessing uses a snake biting its tail as a metaphor for the Rawlings' marriage: an image of something ineffectual, sterile, and utterly pointless. Later on in the story, as she begins her slow, tragic descent into insanity, Susan has a vision of a strange man in her garden, poking what appears to be some kind of snake with a stick. It could be argued that the snake symbolizes Susan's true identity, which has been hidden all these years by her marriage and the subordinate role she's played in it. For too long, Susan's sense of womanhood, her sense of self has been suppressed. But no more. The snake's angry response to being poked is suggestive of Susan's awakening to the reality of her situation, which leads directly to her tragic demise.