Apparatus comprising: a radially-expandable stent, which, when unconstrained in a radially-expanded state, is generally tubular and shaped so as to define: one or more tether interfaces at one or more tether circumferential locations, respectively, each of which tether interfaces extends circumferentially contiguously around less than 30 degrees of a circumference of the stent, and a lower-securement portion that extends (a) along at least a contiguous lower-securement axial segment of the stent and (b) circumferentially around a contiguous lower-securement circumferential portion of the stent, which lower-securement axial segment and lower-securement circumferential portion include the one or more tether interfaces; one or more tissue anchors; and one or more tethers having respective first longitudinal portions that are coupled to the one or more tether interfaces, respectively, and respective second longitudinal portions, different from the respective first longitudinal portions first longitudinal portions, which are coupled to the one or more tissue anchors, respectively, characterized in that the stent, when unconstrained in the radially-expanded state, is shaped so as to further define: a higher-securement portion that extends (a) along at least a contiguous higher-securement axial segment of the stent and (b) circumferentially around between 215 and 330 degrees of the circumference, at all circumferential locations other than those of the lower-securement circumferential portion, and a plurality of outward protrusions at respective circumferential locations around the higher-securement portion, and not around the lower-securement portion. a radially-expandable stent, which, when unconstrained in a radially-expanded state, is generally tubular and shaped so as to define: one or more tether interfaces at one or more tether circumferential locations, respectively, each of which tether interfaces extends circumferentially contiguously around less than 30 degrees of a circumference of the stent, and a lower-securement portion that extends (a) along at least a contiguous lower-securement axial segment of the stent and (b) circumferentially around a contiguous lower-securement circumferential portion of the stent, which lower-securement axial segment and lower-securement circumferential portion include the one or more tether interfaces; one or more tether interfaces at one or more tether circumferential locations, respectively, each of which tether interfaces extends circumferentially contiguously around less than 30 degrees of a circumference of the stent, and a lower-securement portion that extends (a) along at least a contiguous lower-securement axial segment of the stent and (b) circumferentially around a contiguous lower-securement circumferential portion of the stent, which lower-securement axial segment and lower-securement circumferential portion include the one or more tether interfaces; one or more tissue anchors; and one or more tethers having respective first longitudinal portions that are coupled to the one or more tether interfaces, respectively, and respective second longitudinal portions, different from the respective first longitudinal portions first longitudinal portions, which are coupled to the one or more tissue anchors, respectively, characterized in that the stent, when unconstrained in the radially-expanded state, is shaped so as to further define: a higher-securement portion that extends (a) along at least a contiguous higher-securement axial segment of the stent and (b) circumferentially around between 215 and 330 degrees of the circumference, at all circumferential locations other than those of the lower-securement circumferential portion, and a plurality of outward protrusions at respective circumferential locations around the higher-securement portion, and not around the lower-securement portion. a higher-securement portion that extends (a) along at least a contiguous higher-securement axial segment of the stent and (b) circumferentially around between 215 and 330 degrees of the circumference, at all circumferential locations other than those of the lower-securement circumferential portion, and a plurality of outward protrusions at respective circumferential locations around the higher-securement portion, and not around the lower-securement portion.