Apparatus for use with a subject, the apparatus comprising: a catheter: configured to be transluminally advanced toward an anatomical site of the subject, and having a proximal end, a steerable distal end, and a longitudinal axis therebetween; a tissue anchor, configured to be advanced through the catheter; and an anchor driver: having a distal end that is reversibly couplable to the tissue anchor, and configured to drive the tissue anchor through the catheter and out of the distal end of the catheter, and to anchor the tissue anchor at the anatomical site by driving the tissue anchor into tissue at the anatomical site; characterized by: a first constraining member (1602, 1652, 2244) configured: to engage tissue of the subject, and to inhibit, after the anchor has been driven out of the distal end of the catheter and before the anchor has been driven into the tissue at the anatomical site, movement of at least the distal end of the anchor driver, along a first axis between the distal end of the anchor driver and a site at which the first constraining member engages the tissue of the subject; and a second constraining member configured to inhibit, after the anchor has been driven out of the distal end of the catheter and before the anchor has been anchored, movement of at least the distal end of the anchor driver, along a second axis that is different from the first axis. a catheter: configured to be transluminally advanced toward an anatomical site of the subject, and having a proximal end, a steerable distal end, and a longitudinal axis therebetween; configured to be transluminally advanced toward an anatomical site of the subject, and having a proximal end, a steerable distal end, and a longitudinal axis therebetween; a tissue anchor, configured to be advanced through the catheter; and an anchor driver: having a distal end that is reversibly couplable to the tissue anchor, and configured to drive the tissue anchor through the catheter and out of the distal end of the catheter, and to anchor the tissue anchor at the anatomical site by driving the tissue anchor into tissue at the anatomical site; having a distal end that is reversibly couplable to the tissue anchor, and configured to drive the tissue anchor through the catheter and out of the distal end of the catheter, and to anchor the tissue anchor at the anatomical site by driving the tissue anchor into tissue at the anatomical site; a first constraining member (1602, 1652, 2244) configured: to engage tissue of the subject, and to inhibit, after the anchor has been driven out of the distal end of the catheter and before the anchor has been driven into the tissue at the anatomical site, movement of at least the distal end of the anchor driver, along a first axis between the distal end of the anchor driver and a site at which the first constraining member engages the tissue of the subject; and to engage tissue of the subject, and to inhibit, after the anchor has been driven out of the distal end of the catheter and before the anchor has been driven into the tissue at the anatomical site, movement of at least the distal end of the anchor driver, along a first axis between the distal end of the anchor driver and a site at which the first constraining member engages the tissue of the subject; and a second constraining member configured to inhibit, after the anchor has been driven out of the distal end of the catheter and before the anchor has been anchored, movement of at least the distal end of the anchor driver, along a second axis that is different from the first axis.