code: func (c *Constraint) matchesAtMostOneBlob() blob.Ref {
  	if c == nil {
  		return blob.Ref{}
  	}
  	if c.BlobRefPrefix != "" {
  		br, ok := blob.Parse(c.BlobRefPrefix)
  		if ok {
  			return br
  		}
  	}
  	if c.Logical != nil && c.Logical.Op == "and" {
  		if br := c.Logical.A.matchesAtMostOneBlob(); br.Valid() {
  			return br
  		}
  		if br := c.Logical.B.matchesAtMostOneBlob(); br.Valid() {
  			return br
  		}
  	}
  	return blob.Ref{}
  }
docs: // matchesAtMostOneBlob reports whether this constraint matches at most a single blob.
  // If so, it returns that blob. Otherwise it returns a zero, invalid blob.Ref.