A catheter, advanced toward an anatomical site, has a proximal end and a steerable distal end. An anchor is advanced through the catheter. An anchor driver drives the anchor out of the catheter's distal end, anchoring the anchor at the site. A first constraining member engages tissue, and inhibits, after the anchor has been driven out of the catheter and before the anchoring, movement of at least the anchor driver's distal end, on a first axis between the anchor driver's distal end and a site at which the first constraining member engages the tissue. A second constraining member inhibits, after the anchor has been driven out of the catheter and before the anchoring, movement of at least the anchor driver's distal end, on a second axis. Other embodiments are also described.